CO2 sequestration is the process of capturing carbon dioxide and storing it somewhere it won’t reach the atmosphere, whether that’s deep underground, in soil, in trees, or locked into solid rock. It’s one of the core strategies for reducing the amount of greenhouse gas driving climate change. As of early 2025, just over 50 million tonnes of CO2 capture and storage capacity is in operation worldwide, a figure that’s growing but still represents a fraction of the roughly 37 billion tonnes humans emit each year.
Geologic vs. Biologic Sequestration
There are two broad categories. Geologic sequestration stores CO2 in underground rock formations. The gas is typically pressurized until it becomes liquid, then injected into porous rock layers deep beneath the surface, often in saline aquifers (salt-water-saturated rock) or depleted oil and gas fields. Once underground, the CO2 is trapped by four mechanisms: the physical cap of rock above it, surface tension in tiny pore spaces, dissolving into surrounding water, and slowly reacting with minerals to form solid carbonates.
Biologic sequestration stores carbon in living systems: forests, soils, wetlands, and aquatic environments. Trees pull CO2 from the air through photosynthesis and lock it into wood. Soil microbes convert plant material into organic carbon that persists in the ground. Coastal ecosystems like mangroves and seagrass beds are particularly effective at burying carbon in waterlogged sediment where it decomposes very slowly.
The key difference is permanence. Soils may hold carbon for decades to centuries, but a forest fire or a change in land use can release it back quickly. Deep geologic storage, by contrast, can keep CO2 locked away for tens of thousands of years or longer.
How Carbon Mineralization Works
The most permanent form of sequestration turns CO2 into rock. When CO2 dissolves in water, it forms a weak acid. Injected into the right kind of rock, particularly basalt, that acid reacts with calcium, magnesium, and iron in the minerals. The end products are solid carbonate minerals: essentially limestone and its cousins. Once the CO2 is a solid, it’s not going anywhere.
The CarbFix project in Iceland demonstrated this can happen surprisingly fast. Researchers injected CO2 dissolved in water into basalt, and 95% of it mineralized into solid calcite within two years. Since 2014, the project has been capturing and injecting about 12,000 tonnes of CO2 annually from a nearby geothermal plant. A parallel pilot project in Washington State, the Wallula Basalt Pilot Project, injected about 977 tonnes of CO2 into basalt in 2013. By 2015, core samples confirmed that 60 to 65% of it had converted into solid carbonate minerals.
Direct Air Capture
Most sequestration projects capture CO2 at its source, like a power plant or cement factory, where concentrations are high. Direct air capture (DAC) pulls CO2 straight from the ambient atmosphere, where it exists at only about 420 parts per million. That dilution makes the process energy-intensive: current DAC systems require roughly 1,500 to 3,000 kilowatt-hours of energy per tonne of CO2 captured.
Cost is the major barrier. At large scale, estimates range from $100 to $1,350 per tonne of CO2. At smaller, earlier-stage operations, the range climbs to $250 to $1,500 per tonne. The widely cited goal is to bring costs down to $100 per tonne, which would make DAC competitive with other climate strategies. For context, a round-trip flight from New York to London produces roughly one tonne of CO2 per passenger.
Soil Carbon and Farming Practices
Farmland covers a huge share of the planet’s surface, which makes soil carbon sequestration a high-potential strategy even at modest rates per acre. The amount of carbon soil can absorb depends heavily on how the land is managed. Research on regenerative agriculture has quantified how different practices compare, measured in tonnes of organic carbon stored per hectare per year.
On cropland, the most effective individual practices include agroforestry (planting trees alongside crops) at about 1.22 tonnes per hectare per year, and planting double cover crops (one legume, one non-legume) at 1.20 tonnes. No-till farming alone stores roughly 0.48 tonnes per hectare per year, and basic cover cropping about 0.58 tonnes. But combinations matter: pairing cover crops with no-till farming pushes the rate up to about 1.01 tonnes, higher than either practice alone.
On land with woody perennials like vineyards and orchards, integrating grazing animals showed the highest rates at about 2.05 tonnes per hectare per year. Combining cover crops with no-till reached 1.43 tonnes. Researchers identified “quick wins” for farmers: practices that sequester the most carbon while also being relatively easy to adopt. Cover cropping with no-till, non-chemical pest management, and double cover crops all fell into this category.
Utilization: Turning CO2 Into Products
Not all captured CO2 goes into permanent storage. A growing number of projects use it as a raw material. Captured CO2 can be converted into synthetic fuels, fed into chemical processes to make polyols (a building block for plastics and foams), used as food-grade CO2 for carbonated drinks, or injected into concrete where it mineralizes and actually strengthens the material.
A study modeling different uses for CO2 captured from a cement plant found that producing polyols could avoid about 708,000 tonnes of CO2 per year while generating a profit of roughly 18 euros per tonne of CO2 avoided. Food-grade CO2 production was feasible but costlier, at about 108 euros per tonne avoided, and only when it replaced CO2 derived from fossil sources. One important caveat: in the scenarios studied, less than 10% of the plant’s total emissions could be directed to utilization. The rest still needed conventional underground storage.
Risks of Underground Storage
The main concern with geologic sequestration is leakage. CO2 could escape through three pathways: active injection wells, abandoned wells from past oil and gas drilling, and natural fractures or faults in the rock. Of these, abandoned wells are considered the most significant hazard because they’re numerous in many regions, their condition is often poorly documented, and their cement seals may have degraded over decades.
Monitoring techniques include pulsed neutron logging (which detects fluids in rock), noble gas tracers that can fingerprint injected CO2 if it migrates, and oxygen isotope analysis. A key uncertainty is that we lack long-term empirical data on how CO2 behaves underground over thousands of years. Modeling suggests the risk is manageable, but regulators improve storage security most effectively by identifying and monitoring abandoned wells and being prepared to remediate any that leak.
How Long Each Method Lasts
Permanence varies enormously by method, and it matters because CO2 released back into the atmosphere after 50 years of storage hasn’t solved anything. Soil carbon typically persists for decades to centuries, depending on land management. A change in farming practices, drought, or fire can reverse those gains. Forest carbon faces similar vulnerabilities.
Deep saline aquifer storage is expected to last tens of thousands of years, well beyond the timescale that matters for climate. Carbon mineralization is the gold standard: once CO2 is a solid mineral in rock, it’s stable on geologic timescales of millions of years. Climate scientists have argued that sequestration should be considered permanent only if it lasts at least 10,000 years, which effectively rules out biological methods as standalone solutions and points toward geologic storage and mineralization as the most reliable long-term approaches.