Carbon dioxide can be removed from air through several proven methods, ranging from industrial machines that chemically filter it out to natural processes like forest growth and rock weathering. The challenge isn’t whether it’s possible, but doing it affordably and at a scale that matters. Nature currently absorbs roughly 22.7 billion metric tons of CO2 per year through oceans and land ecosystems, while all technological removal methods combined captured only about 40,000 metric tons in 2023.
Direct Air Capture Machines
Direct air capture (DAC) is the most talked-about technological approach. Large fans pull ambient air through a system containing chemical sorbents, materials designed to grab CO2 molecules and hold them. Once the sorbent is saturated, heat is applied to release the concentrated CO2 for storage or use. The sorbent is then recycled and the process repeats.
There are two main types. Liquid solvent systems pass air through a solution (typically potassium hydroxide) that reacts with CO2 to form a carbonate compound. Releasing the captured CO2 requires heating to around 900°C, which demands significant energy. Newer liquid approaches use different chemistry to bring that regeneration temperature down below 120°C, dramatically cutting energy needs.
Solid sorbent systems work differently. CO2 molecules stick to the surface of specially engineered materials like metal-organic frameworks or zeolites. These systems typically release their CO2 at lower temperatures than liquid solvents, requiring 4 to 6 gigajoules of energy per ton of CO2 captured, compared to 6 to 10 gigajoules for liquid systems. In practice, a DAC plant uses roughly 1,500 kilowatt-hours of heat and 500 kilowatt-hours of electricity to capture a single ton of CO2, with total energy consumption ranging from 2,000 to 3,000 kilowatt-hours per ton.
The biggest barrier is cost. Current DAC projects run between $500 and $1,900 per ton of CO2 removed. Industry projections suggest costs could fall to around $300 per ton by mid-century, with some next-generation designs targeting the $100 mark. For context, humanity emits over 35 billion tons of CO2 annually.
Turning CO2 Into Stone
Once CO2 is captured, it needs to go somewhere permanent. The most durable solution is mineralization: injecting CO2 dissolved in water deep into basalt rock formations underground. The CO2 forms carbonic acid, which reacts with minerals in the basalt (olivine, pyroxene, and feldspar) to release calcium, magnesium, and iron. These elements then combine with the carbon to form solid carbonate minerals, essentially turning the gas into rock.
This isn’t theoretical. The CarbFix project in Iceland has demonstrated that 95% of injected CO2 mineralizes into calcite within two years. The Wallula Basalt Pilot Project in Washington state confirmed similarly rapid results. Once mineralized, the carbon is locked away permanently, not for centuries, but for geological timescales.
Enhanced Rock Weathering
Rain naturally breaks down rocks over thousands of years, releasing calcium and magnesium that eventually bind with atmospheric CO2 to form new minerals like limestone. Enhanced rock weathering speeds this up by grinding rocks, particularly basalt and olivine, into fine dust and spreading it across farmland or coastlines where it reacts with CO2 far faster than intact rock would.
Olivine weathers so quickly in natural conditions that it’s rare on Earth’s surface despite being abundant below the crust. Crushing it into powder dramatically increases the surface area exposed to air and water, accelerating the chemical reactions. Researchers are also experimenting with adding catalysts or even bacteria and lichens to speed up the carbon-binding process further. The approach has the added benefit of reducing soil acidity and potentially improving crop yields.
Ocean-Based Removal
The ocean already absorbs about 3 billion metric tons of carbon per year, making it Earth’s largest active CO2 sink. Several approaches aim to boost that capacity.
Alkalinity enhancement involves adding alkaline minerals to seawater to shift its chemistry so it absorbs more CO2 from the atmosphere. The CO2 converts into stable carbonate forms that remain dissolved, which also counteracts ocean acidification. Some projects combine this with electrolysis-driven mineral weathering powered by wave energy.
Kelp farming takes a biological approach. Kelp grows rapidly, pulling CO2 from surface waters as it builds tissue. When the kelp dies and sinks to the deep ocean floor, the carbon it contains gets buried in sediments. Iron fertilization, which adds trace amounts of iron to nutrient-poor ocean regions to stimulate plankton growth, follows similar logic but remains more controversial due to potential unintended ecological effects.
Biochar and Soil Carbon Storage
Biochar is made by heating organic material (wood, crop waste, manure) in a low-oxygen environment, a process called pyrolysis. The result is a carbon-rich, charcoal-like substance with a highly stable molecular structure that resists microbial breakdown. When mixed into soil, it locks carbon away while also improving soil health.
How long the carbon actually stays put depends heavily on soil type. Long-term field experiments in Germany tracked biochar applied 12 to 14 years ago and found strikingly different results depending on where it was used. In loamy soil, biochar additions increased soil carbon stocks by 38 metric tons per hectare, and that increase remained stable after 11 years. In sandy soil, the picture was far less encouraging: an initial increase of 61 metric tons per hectare dropped by 38 metric tons within four years, likely because the coarse soil couldn’t physically protect the biochar from erosion and decomposition. After nine years, the sandy soil held only slightly more carbon than untreated plots. The takeaway is that biochar works best in clay-rich or loamy soils that give it physical protection.
Trees and Natural Land Sinks
Forests remain the most accessible and well-understood carbon removal tool. Trees pull CO2 from the air during photosynthesis, storing carbon in their wood, roots, leaves, and the surrounding soil. Earth’s land ecosystems collectively absorbed an estimated 3.2 billion metric tons of carbon in 2024, though this number fluctuates year to year depending on drought, fire seasons, and land-use changes.
Planting new forests (afforestation) and restoring degraded ones (reforestation) are relatively low-cost compared to technological removal, typically costing $10 to $50 per ton of CO2. The tradeoff is permanence: a forest fire, disease outbreak, or future logging decision can release stored carbon back into the atmosphere within days. Trees also take decades to reach their peak carbon absorption rates, making forests a slow-building but powerful tool when protected long-term.
How These Methods Compare
No single approach can solve the problem alone. Each method sits on a spectrum between cost, permanence, and scalability:
- Direct air capture: Highly permanent when paired with underground mineralization, but expensive ($500 to $1,900 per ton) and energy-intensive. Currently operating at tiny scale.
- Enhanced rock weathering: Lower cost and uses abundant materials, but difficult to measure precisely and works over years rather than instantly.
- Ocean alkalinity enhancement: Enormous theoretical capacity given the ocean’s size, but ecological risks are not fully understood.
- Biochar: Relatively affordable and beneficial for agriculture, but long-term storage depends on soil conditions.
- Reforestation: Cheapest and most immediately deployable, but carbon storage is vulnerable to fire, disease, and human decisions.
The gap between what technology removes and what nature removes is enormous. Technological carbon removal captured roughly 40,000 metric tons of CO2 in 2023. Natural sinks absorbed over 22 billion metric tons. Closing that gap will require scaling every available method simultaneously, not choosing one winner.