Carbon dioxide removal, or CDR, refers to any process that pulls CO₂ out of the atmosphere and locks it away in some form of storage. The main methods span a wide spectrum: engineered systems like direct air capture and bioenergy with carbon capture, nature-based approaches like reforestation and soil carbon management, geochemical techniques like enhanced rock weathering, and ocean-based strategies. Keeping warming well below 2°C likely requires current CDR capacity to grow at least exponentially through the end of this decade, adding a minimum of 300 million tonnes of CO₂ removal capacity.1Environmental Research Letters. Quantifying global carbon dioxide removal deployment No single method can do the job alone, and each comes with its own set of costs, energy demands, and environmental trade-offs worth understanding.
Direct Air Capture
Direct air capture, or DAC, uses chemical processes to strip CO₂ directly from ambient air. The two leading approaches differ in how they grab the CO₂. Liquid solvent systems pump air through a solution, typically a potassium hydroxide solution, that binds with CO₂. The captured carbon then goes through a series of steps: precipitation, heating in a high-temperature kiln to release a concentrated CO₂ stream, and finally compression for transport and storage.2Communications Earth & Environment. Liquid solvent direct air capture’s cost and carbon dioxide removal vary with ambient environmental conditions Solid sorbent systems take a different approach, passing air over solid materials that chemically bind CO₂ at low temperatures and then release it when heated. Comparative assessments have found that the solid sorbent process has the potential to offer the best thermodynamic performance of the DAC technologies studied.3Joule. Comparative technical and economic assessment of direct air capture technologies
The appeal of DAC is that it can work almost anywhere, does not compete with food production for land, and delivers CO₂ in a concentrated form ready for permanent geological storage. The drawback is energy. Both liquid and solid sorbent systems need substantial heat and electricity. Current costs remain high compared to other CDR methods, though they have been falling as pilot and commercial plants scale up. Performance also varies with local conditions: humidity, temperature, and wind speed all affect how efficiently a DAC plant captures CO₂ and how much water it loses through evaporation.
Bioenergy with Carbon Capture and Storage
Bioenergy with carbon capture and storage, or BECCS, works on a two-step logic. Plants absorb CO₂ as they grow. When those plants are burned for energy, the CO₂ released at the power plant or biorefinery is captured before it reaches the atmosphere and piped underground for permanent storage. In theory, the result is net-negative emissions: CO₂ is removed from the atmosphere by photosynthesis and then locked away geologically.
Climate models rely heavily on BECCS. One set of modeled scenarios projects hundreds of billions of tonnes of cumulative land-based CDR under a 2°C pathway, with BECCS contributing the lion’s share.4Nature Communications. Trade-offs in land-based carbon removal measures under 1.5 °C and 2 °C futures But the real-world sustainability of large-scale BECCS is far less certain. Growing enough biomass to feed these systems would require vast amounts of land, water, and fertilizer. One analysis found that when you account for the pressure BECCS plantations place on biodiversity, freshwater systems, and land use, the potential for dedicated energy crops like miscanthus drops close to zero under realistic environmental constraints.5Communications Earth & Environment. Multiple planetary boundaries preclude biomass crops for carbon capture and storage outside of agricultural areas The sustainability picture improves considerably if BECCS is limited to waste and residue feedstocks rather than purpose-grown crops.6Global Policy. Managing Land‐based CDR: BECCS, Forests and Carbon Sequestration The tension between how much BECCS climate models assume and how much the planet can actually sustain is one of the sharpest disagreements in the CDR field.
Afforestation and Reforestation
Planting new forests on previously unforested land (afforestation) or restoring forests where they once stood (reforestation) is the most intuitive form of CDR: trees absorb CO₂ as they grow, storing carbon in their wood, roots, and the soil around them. Over long simulation periods, plantations left unharvested generate the greatest potential for carbon storage in ecosystems. When trees are harvested, it takes decades to recover the lost carbon, and that recovery only works out favorably if a significant portion of the harvested wood ends up in long-lived products like structural timber.7New Forests. Carbon sequestration and emission mitigation potential of afforestation and reforestation of unproductive territories
The vulnerability of forests as a carbon store is the elephant in the room. The carbon locked up in trees has to stay there for decades to deliver meaningful climate benefits, and forests face growing threats from wildfire, drought, pests, and disease, all of which are worsening with climate change. Policies aimed at boosting forest carbon need to account for these risks of premature carbon release.8Forest Policy and Economics. Carbon sequestration through afforestation under uncertainty A forest that burns down returns its stored carbon to the atmosphere in a matter of days, and the frequency and severity of these events is increasing. This is a fundamentally different risk profile from, say, CO₂ injected into deep geological formations, where the storage is essentially permanent.
Soil Carbon Sequestration
Agricultural soils hold enormous amounts of carbon, and management practices like planting cover crops or reducing tillage can increase how much carbon sticks around. A meta-analysis of cover crop studies estimated that soils could accumulate an average of about 16.7 tonnes of carbon per hectare over a modeled period of roughly 155 years of continuous cover crop cultivation before reaching a new equilibrium.9Agriculture, Ecosystems & Environment. Carbon sequestration in agricultural soils via cultivation of cover crops – A meta-analysis That is a slow process, but across millions of hectares of farmland, the numbers add up.
There are hard limits, though. Soil has a finite capacity to hold carbon. Long-term no-till experiments have shown that the fine mineral particles in topsoil, the fraction that stabilizes carbon most effectively, can approach saturation. In one study, the capacity of the top layer to stabilize additional carbon dropped by over 90% compared to bare soil, with most of the remaining storage capacity found in deeper layers.10Geoderma. Carbon sequestration capacity in no-till soil decreases in the long-term due to saturation of fine silt plus clay-size fraction Broader analysis across multiple field experiments confirms that soil carbon does saturate: soils that are already carbon-rich respond less to additional inputs, meaning the greatest efficiency gains come from restoring carbon in depleted soils.11Biogeochemistry. Soil carbon saturation: concept, evidence and evaluation Soil carbon is also reversible. If a farmer switches back to conventional tillage or stops planting cover crops, the accumulated carbon can be lost over years to decades. This makes soil carbon a valuable but temperamental carbon store.
Biochar
Biochar is made by heating organic material, anything from wood waste to crop residues to manure, in a low-oxygen environment through a process called pyrolysis. The resulting charcoal-like substance resists decomposition and can be mixed into soil, locking carbon away for long periods. Estimated mean residence times for biochar carbon in soil range from roughly 90 to 1,600 years depending on the feedstock and production temperature.12Environmental Science & Technology. Biochar Carbon Stability in a Clayey Soil As a Function of Feedstock and Pyrolysis Temperature That puts biochar in an interesting middle ground: it is far more durable than forest or soil carbon but leaks slowly and predictably rather than storing carbon permanently the way geological injection does.
Production conditions matter a great deal. Biochar made at higher temperatures tends to be more stable because the carbon becomes more chemically resistant. Research on spent mushroom substrate showed that carbon sequestration in biochar increased from about 16% at 300°C to about 42% at 600°C, and that adding mineral iron boosted the sequestration rate further to about 61%.13PubMed Central. Effect of Pyrolysis Temperature on the Carbon Sequestration Capacity of Spent Mushroom Substrate Biochar in the Presence of Mineral Iron Similar work with calcium additives found that the maximum carbon sequestration was achieved at the highest pyrolysis temperature tested when calcium was incorporated into the feedstock.14PubMed. Pyrolysis temperature-dependent carbon retention and stability of biochar with participation of calcium: Implications to carbon sequestration Beyond carbon storage, biochar can improve soil fertility and water retention, which gives it practical appeal for farmers. The challenge is scaling production to climatically meaningful volumes without competing for biomass that could serve other purposes.
Blue Carbon Ecosystems
Coastal ecosystems like mangrove forests, salt marshes, and seagrass meadows store carbon at rates that dwarf most terrestrial forests per unit area. They trap organic matter in waterlogged sediments where decomposition is slow, building up carbon deposits over centuries. Recent work has expanded the blue carbon picture by quantifying the contribution of unvegetated tidal flats, which sequester an estimated 8.6 million tonnes of carbon per year and store roughly 1.2 billion tonnes in the top meter of their sediments, increasing global blue carbon estimates by 6% to 17%.15PubMed. Unvegetated Tidal Flats: A Critical Yet Vulnerable Coastal Blue Carbon Sink
The picture gets complicated by methane. When organic matter decomposes in the oxygen-poor sediments of these ecosystems, some of it is converted to methane, a greenhouse gas far more potent than CO₂ over short time horizons. In mangrove forests, methane emissions can offset blue carbon burial rates by about 20% on a 20-year warming basis.16PubMed Central. Methane emissions partially offset “blue carbon” burial in mangroves Tidal flats face a similar issue, with methane emissions offsetting roughly 14% of their carbon sequestration benefit.15PubMed. Unvegetated Tidal Flats: A Critical Yet Vulnerable Coastal Blue Carbon Sink None of this negates the value of protecting and restoring coastal ecosystems, but it does mean that headline carbon storage numbers overstate the net climate benefit if methane is ignored.
Restoration is promising but takes time. A meta-analysis of managed blue carbon sites found that restored sites showed significant increases in blue carbon after about four years compared to degraded sites, with the potential to reach carbon stocks comparable to natural sites after 7 to 17 years.17PubMed. Impacts of land management practices on blue carbon stocks and greenhouse gas fluxes in coastal ecosystems-A meta-analysis The strongest evidence supports straightforward restoration approaches; alternative management techniques like sediment manipulation showed weaker results for soil carbon stocks.
Enhanced Rock Weathering
Over geological timescales, the natural weathering of silicate rocks is one of the planet’s primary mechanisms for drawing down atmospheric CO₂. Enhanced rock weathering, or ERW, tries to speed this process up by grinding rocks like basalt into fine particles and spreading them on agricultural fields. As rainwater dissolves the rock, chemical reactions convert CO₂ into dissolved bicarbonates that eventually wash into rivers and the ocean, where the carbon can remain locked away for thousands of years.
Field evidence is accumulating, though the picture is mixed. A hillslope-scale test measuring actual CO₂ removal from basalt dissolution found removal rates in the range reported across the broader literature, but also identified complications: CO₂ levels dropped as water infiltrated the soil, and the rock did not dissolve evenly.18Earth and Planetary Science Letters. Carbon dioxide removal during dissolution of granular basalt: A mass balance test of enhanced rock weathering at the hillslope scale A UK-based soil core study found that basalt application did increase dissolved calcium, magnesium, and other elements consistent with rock weathering, and saw evidence of secondary carbonate formation deeper in the soil, which is a sign that carbon is being stored. But the study also flagged limitations in dry cropland conditions.19Applied Geochemistry. Soil core study indicates limited CO2 removal by enhanced weathering in dry croplands in the UK Climate and soil moisture appear to be key variables: ERW works better in warm, wet conditions where dissolution proceeds faster. Dry regions may see minimal benefit.
One of ERW’s practical advantages is its compatibility with existing agriculture. Farmers already apply lime and other amendments to their fields, and crushed basalt can serve a similar function while also providing nutrients like calcium and magnesium. The co-benefits could help drive adoption, but verifying exactly how much CO₂ any given application removes remains a significant measurement challenge.
Ocean Alkalinity Enhancement and Iron Fertilization
The ocean already absorbs roughly a quarter of human CO₂ emissions each year, and several CDR strategies aim to boost that capacity. Ocean alkalinity enhancement involves adding alkaline materials to seawater to increase its ability to absorb and hold CO₂. One approach involves the electrochemical processing of calcium carbonate from limestone to produce dissolved hydroxides, which absorb and neutralize CO₂ while also potentially counteracting ocean acidification.20PubMed. Electrochemical splitting of calcium carbonate to increase solution alkalinity: implications for mitigation of carbon dioxide and ocean acidity The resulting dissolved bicarbonate acts as the carbon storage medium, diluted within the ocean itself.
Iron fertilization takes a biological angle. Large swaths of the ocean are nutrient-limited: they have the sunlight and CO₂ to support more photosynthesis but lack trace nutrients like iron. Adding iron stimulates phytoplankton blooms, which absorb CO₂ at the surface. Some of that carbon then sinks to the deep ocean as dead organisms fall. Modeling of patch-scale experiments found high atmospheric uptake efficiencies, with ratios of 0.75 to 0.93 between the CO₂ drawn from the atmosphere and the carbon exported below 100 meters over a ten-year period. But the total amount of CO₂ removed from the atmosphere was small at the scales tested.21Biogeosciences. The impact on atmospheric CO2 of iron fertilization induced changes in the ocean’s biological pump Ecological side effects, including potential disruption of marine food webs and the creation of oxygen-depleted zones, remain concerns that have kept iron fertilization controversial and largely confined to research settings.
How Long the Carbon Stays Removed
Permanence is the dimension that separates CDR methods most starkly. Geological storage, the kind used in BECCS or DAC paired with underground injection, holds CO₂ on timescales of thousands to millions of years. Biochar leaks slowly and predictably. Forests and soils can release their stored carbon quickly through fire, drought, land-use change, or a shift in farming practice.22Joule. Carbon Dioxide Removal: What Are the Main Methods?
This matters enormously for whether CDR actually stabilizes the climate. Modeling shows that if CDR used to offset hard-to-eliminate emissions has a mean storage duration of only 100 years, it can lead to an additional 0.8°C of warming over 400 years compared to permanent removal, even under a scenario with 6 billion tonnes per year of residual emissions.23Communications Earth & Environment. Durability of carbon dioxide removal is critical for Paris climate goals In other words, impermanent removal is not worthless, but treating it as equivalent to permanent removal in climate accounting leads to dangerous overconfidence. A tonne of CO₂ stored geologically for millennia is not the same climate product as a tonne stored in a forest that might burn down in 30 years.
Resource Trade-offs at Scale
Every CDR method needs something: energy, land, water, minerals, or some combination. The resource demands become acute when you scale methods up to the billions of tonnes per year that climate models envision. Scenario modeling for Asia found that a high-CDR pathway could consume about 15 cubic kilometers of water per year for DAC alone by mid-century, plus an additional 3.6 cubic kilometers per year for bioenergy CCS, compared to 1.7 cubic kilometers under a more moderate deployment scenario.24Nature Communications. Deployment expectations of multi-gigatonne scale carbon removal could have adverse impacts on Asia’s energy-water-land nexus Fertilizer demand for bioenergy crops also rises sharply in high-CDR scenarios, competing with food production for both nutrients and land.
These are not reasons to abandon CDR but reasons to be realistic about what any single method can deliver. A portfolio approach, combining multiple methods at moderate scales, is likely to be less disruptive than betting everything on one technology. Land-based biological methods work best as part of broader agricultural and conservation goals rather than as dedicated carbon factories. Engineered methods avoid land-use conflicts but need clean energy to avoid simply reshuffling emissions from one sector to another. Matching CDR methods to local conditions and resources, rather than assuming universal scalability, is the practical path forward.
The Mitigation Deterrence Problem
There is a real concern in the policy world that the promise of future CDR could give governments and industries an excuse to delay cutting emissions now. If policymakers assume they can remove large amounts of CO₂ later, they may allow higher emissions today, banking on CDR that might never materialize at the necessary scale. Multiple scholars have pushed for separating emission reduction targets from CDR targets to avoid this trap, arguing that treating a tonne of avoided emissions as interchangeable with a tonne of future removal is especially dangerous if CDR methods fail to deliver.25Nature. Separating CO2 emission from removal targets comes with limited cost impacts The concern is not hypothetical: climate models that lean heavily on BECCS to hit temperature targets have already been criticized for assuming deployment scales that may be physically or ecologically impossible.
None of this means CDR is unnecessary. Even the most aggressive emission reduction pathways leave residual emissions from sectors like aviation, cement, and agriculture that are extremely difficult to decarbonize fully. CDR is how those residual emissions get balanced. The argument is about sequencing and honesty: cut emissions first and fastest, deploy CDR to handle what remains, and do not let the promise of future removal slow down today’s cuts.
Community Acceptance and Environmental Justice
The physical infrastructure of CDR, whether it is CO₂ pipelines, injection wells, or large biomass plantations, has to go somewhere. Where it goes tends to follow familiar patterns: communities that are already burdened by pollution and economic disadvantage are often the ones asked to host new industrial facilities. Survey work in California’s Delta region and Kern County explored how communities responded to proposed carbon capture and geological storage projects. Support for new CO₂ infrastructure, including pipeline construction, well drilling, injection, and storage, was below 50% in both areas.26The Electricity Journal. Environmental and climate justice and technological carbon removal Respondents consistently highlighted air quality, water quality, jobs, and the local economy as their top concerns. In Kern County, where the oil industry already has a strong presence, some respondents were more optimistic about the economic opportunity but still wanted strict environmental protections.
The findings reinforce a straightforward point: CDR projects are more likely to gain acceptance when they provide tangible local benefits, permanent jobs rather than temporary construction work, and meaningful environmental safeguards. Communities that have spent decades breathing polluted air and drinking compromised water are understandably skeptical of promises that a new industrial facility will be different this time. Inclusive public engagement and genuine decision-making power for affected communities are not nice-to-haves. They are practical prerequisites for deployment at scale.
Locking Carbon into Building Materials
One CDR pathway that is easy to overlook involves turning captured CO₂ into solid products, particularly concrete and building materials. Concrete is already one of the most widely used materials on Earth, and several commercial methods now exist for incorporating CO₂ into it. These include curing fresh concrete or cement with CO₂ gas, carbonating industrial waste products like steel slag into building blocks, producing synthetic aggregates through carbonation, and using carbonated minerals as partial cement replacements.27Journal of CO2 Utilization. Carbon sequestration and storage in concrete: A state-of-the-art review of compositions, methods, and developments Once the CO₂ is mineralized into the concrete, it is stored in a form that is effectively permanent for the life of the structure and beyond, since the carbonate minerals are thermodynamically stable.
The scale of the opportunity is significant simply because the world produces so much concrete. Even modest incorporation of CO₂ per tonne of concrete adds up across global production. The economics are helped by the fact that carbonation can actually improve certain material properties, meaning producers may adopt it for performance reasons as well as carbon reasons. The limitation is that this approach typically stores less CO₂ per tonne than it takes to make the cement in the first place, so it reduces the carbon footprint of concrete rather than making the material net-negative. Still, as one piece of a broader CDR portfolio, mineralization into construction materials has the advantages of permanence, a ready market, and compatibility with existing supply chains.