How Carbon sequestration Works

When sunlight hits a plant leaf, chlorophyll molecules capture that energy and use it to split water molecules and grab carbon dioxide from the air. The plant then combines these ingredients to manufacture glucose sugars, releasing oxygen as a byproduct. This process transforms gaseous carbon into solid plant matter—the wood of tree trunks, the cellulose of grass blades, the starches in roots.

Forests represent massive carbon storage systems, with a single mature tree capable of absorbing roughly 48 pounds of CO2 annually. Tropical rainforests alone store approximately 250 billion tons of carbon in their biomass and soil. When trees die naturally and decompose slowly, or when their carbon gets incorporated into stable soil organic matter, that captured carbon can remain locked away for decades or even centuries.

The scale of plant-based sequestration depends critically on photosynthetic efficiency and ecosystem health. Fast-growing forests sequester carbon more rapidly than old-growth forests, though mature forests store far more total carbon. Agricultural practices like cover cropping and no-till farming can enhance soil carbon storage, while deforestation releases stored carbon back to the atmosphere—turning natural carbon sinks into sources of greenhouse gas emissions.

Carbon capture and storage (CCS) begins at emission sources like coal power plants, natural gas facilities, or industrial factories where CO2 concentrations are high. Chemical solvents or filters separate CO2 from other exhaust gases, concentrating it into a pure stream. This captured carbon is then compressed into a liquid-like supercritical state, reducing its volume and making it transportable through pipelines.

The injection process targets geological formations typically one to two miles below the surface, where porous sedimentary rocks act like rigid sponges with microscopic spaces between mineral grains. Saline aquifers—rock layers saturated with salty water unsuitable for drinking—offer vast storage capacity across many regions. Depleted oil and gas reservoirs provide another option, repurposing the same underground traps that held fossil fuels for millions of years.

Once injected, the supercritical CO2 fills the pore spaces within the rock, displacing the existing brine. A caprock layer of impermeable shale or clay sits above these reservoirs, preventing the carbon from migrating back toward the surface. Over time, the CO2 dissolves into the brine and eventually reacts with minerals to form stable carbonate compounds, providing multiple barriers against leakage. Large-scale projects like Norway's Sleipner field have successfully stored millions of tons of CO2 since the 1990s.

The ocean surface constantly exchanges gases with the atmosphere, absorbing roughly 25-30% of human-generated CO2 emissions each year. When carbon dioxide dissolves in seawater, it reacts with water molecules to form carbonic acid, which then breaks down into bicarbonate and carbonate ions. These dissolved inorganic carbon compounds can persist in ocean waters for hundreds to thousands of years before cycling back to the atmosphere.

Ocean circulation patterns determine how effectively this dissolved carbon gets sequestered long-term. Surface waters mix downward through a process called thermohaline circulation, where cold, dense water near the poles sinks and carries dissolved carbon into the deep ocean. This "biological pump" is enhanced by marine phytoplankton, which absorb CO2 during photosynthesis and transport carbon to deeper waters when they die and sink as organic particles.

However, increasing ocean CO2 absorption comes with consequences for marine chemistry. The carbonic acid formed during dissolution lowers seawater pH, a process called ocean acidification. This chemical shift makes it harder for organisms like corals, mollusks, and certain plankton to build their calcium carbonate shells and skeletons, potentially disrupting marine ecosystems and limiting the ocean's future capacity to absorb carbon.

Mineral carbonation mimics a geological process that normally unfolds over thousands of years, compressing it into months or years. When CO2 contacts certain rocks rich in magnesium or calcium—such as basalt, peridotite, or industrial waste like steel slag—it reacts with these metal oxides to form stable carbonate minerals like limestone. These solid carbonates represent the most permanent form of carbon storage, with essentially zero risk of the carbon returning to the atmosphere.

Two main approaches exist: ex-situ mineralization processes CO2 and crushed rock together in industrial reactors under controlled temperature and pressure, producing solid carbonates that can be used as construction materials or safely landfilled. In-situ mineralization injects CO2 directly into reactive rock formations underground, where it naturally mineralizes over time. Iceland's CarbFix project demonstrated this approach by injecting CO2 into basalt formations, achieving 95% mineralization within just two years.

The challenge lies in reaction speed and resource requirements. Mineralization demands large quantities of suitable rock—roughly two to four tons of silicate minerals per ton of CO2 sequestered. Grinding rock to increase surface area and accelerate reactions requires significant energy. Despite these limitations, mineralization offers unmatched storage security, essentially returning carbon to the geological reservoirs from which fossil fuels originally came, completing a full cycle over dramatically shortened timescales.

Direct air capture (DAC) technology uses large fans to pull ordinary air through specialized filters or chemical solutions that selectively bind with CO2 molecules. Unlike capturing carbon at emission sources where concentrations reach 10-15%, DAC must extract CO2 from air containing just 420 parts per million—roughly 0.04% of the atmosphere. The most common approaches use solid sorbents (materials that absorb gases) or liquid solvents containing amines or hydroxide compounds that chemically bond with carbon dioxide while letting nitrogen, oxygen, and other gases pass through.

After the filters or solutions become saturated with CO2, the system must regenerate them to release the captured carbon as a concentrated stream. Solid sorbent systems typically heat the filters to 80-120°C, breaking the chemical bonds and freeing the CO2. Liquid solvent systems often use hydroxide solutions that form carbonates when exposed to CO2, then apply high heat (900°C) to calcine these carbonates back into hydroxides and pure CO2 gas. This regeneration step consumes substantial energy, representing the primary cost and challenge for DAC deployment.

The concentrated CO2 can then be compressed and either stored underground via injection or used to manufacture products like synthetic fuels, concrete, or chemicals. Companies like Climeworks in Switzerland and Carbon Engineering in Canada operate commercial DAC facilities, though current costs range from $600-1000 per ton of CO2 removed. Unlike natural sequestration methods tied to specific ecosystems or geological formations, DAC systems can theoretically be built anywhere with sufficient renewable energy, offering flexibility to address emissions from diffuse sources like aviation and agriculture that are difficult to decarbonize.