Engineered Removal: DACCS, Biochar, and Enhanced Weathering
Engineered carbon dioxide removal (CDR) technologies occupy an increasingly central position in the science and policy of climate change. The Intergovernmental Panel on Climate Change's Sixth Assessment Report concluded that virtually every pathway to limiting warming to 1.5 degrees Celsius requires significant deployment of CDR by mid-century. The voluntary carbon market has become one of the primary early-stage financing mechanisms for bringing these technologies from laboratory to commercial scale.
Why Engineered Removal Matters
Nature-based solutions sequester carbon in ecosystems that are vulnerable to reversal. Engineered removal technologies aim for more durable storage: carbon locked away in stable geological formations, bound into solid materials, or mineralised into rock. This durability premium is why durable removal credits can command prices an order of magnitude above nature-based avoidance credits, even though their volumes remain tiny by comparison.
The three technologies examined in this lesson sit at different points on the maturity, cost, and permanence spectrum.
Direct Air Carbon Capture and Storage (DACCS)
DACCS uses mechanical and chemical processes to extract CO2 directly from ambient air and then permanently store it underground in geological formations. The process is energy-intensive: large fans or contactors push air over a sorbent material (liquid or solid) that binds CO2. The CO2 is then released through heating, compressed, and injected into underground rock formations where it mineralises over decades.
Key characteristics of DACCS:
- Permanence: Effectively permanent when stored in geological formations. IPCC literature typically assigns >10,000-year storage certainty to properly selected geological sites.
- Current cost: Approximately $300 to $1,000 per tonne of CO2 as of 2024, with leading companies (Climeworks, Carbon Engineering/Oxy) targeting costs below $300 by 2030 through scale-up.
- Energy demand: Current systems require roughly 1,500 to 2,000 kWh of energy per tonne of CO2 captured. This energy must itself be low-carbon for the net removal to be credible.
- Scale: Global DACCS capacity in 2024 was approximately 10,000 tonnes per year across all operating facilities combined - a negligible fraction of the gigatonne scale required by mid-century scenarios.
Example: Climeworks Mammoth Plant, Iceland
Climeworks operates the world's largest DACCS plant in Iceland, powered by geothermal energy (ensuring near-zero net emissions from the energy supply). The Mammoth facility has a design capacity of 36,000 tonnes per year. Microsoft and other early corporate buyers have pre-purchased removal credits from Climeworks at prices reported to exceed $1,000 per tonne, acknowledging both the current cost and the market-development rationale for premium pricing.
Biochar
Biochar is produced by heating biomass (wood waste, agricultural residues, sewage sludge) in a low-oxygen environment (pyrolysis). The resulting charcoal-like material is highly stable and, when applied to agricultural soils, can lock carbon away for hundreds to thousands of years while also improving soil fertility and water retention.
Biochar sits at the intersection of agricultural, forestry, and waste management sectors, making it more practically scalable than DACCS in the near term:
- Permanence: Mean residence time of 100 to 1,000+ years depending on feedstock and soil conditions. Puro.earth and other registries use H/Corg ratios (a measure of aromatic carbon content) to estimate stability.
- Current cost: Approximately $100 to $300 per tonne, already lower than DACCS and declining with scale.
- Co-benefits: Improved soil fertility, water retention, and potential substitution for synthetic fertilisers can provide economic value to farmers that partially offsets biochar production costs.
- MRV approach: Credits are calculated from feedstock weight, pyrolysis conditions, and stability assessments. Some methodologies require soil sampling to verify carbon persistence.
Analogy: Charcoal as a Carbon Time Capsule
When biomass decomposes naturally, the carbon it contains returns to the atmosphere within years to decades. Pyrolysis interrupts this cycle - the carbon in the biomass is transformed into a stable aromatic structure that resists microbial decomposition. The carbon that would have released from decaying crop residue in five years instead stays locked in soil for five centuries. The biochar acts as a carbon time capsule buried in the field.
Enhanced Weathering
Enhanced weathering accelerates the natural geological process by which silicate rocks react with atmospheric CO2 to form stable bicarbonates or carbonate minerals. In practice, projects crush silicate rocks (typically basalt) into fine powder and spread them on agricultural land or in oceans, dramatically increasing the surface area available for the chemical reaction.
Key characteristics of enhanced weathering:
- Permanence: Effectively permanent once carbon is mineralised into bicarbonate ions in the ocean or carbonate rock in soils. No reversal risk under normal conditions.
- Current cost: Estimates range from $80 to $200 per tonne depending on rock source proximity, grinding energy, and application method.
- MRV challenge: Quantifying how much CO2 has actually been captured is technically demanding. The reaction occurs in soils and water over extended periods. Leading approaches include geochemical soil sampling, mass balance models, and measurement of cation release rates. This MRV uncertainty is the primary reason registries have been cautious in certifying enhanced weathering at scale.
- Co-benefits: Basalt application can improve soil pH and provide plant-available nutrients (silicon, calcium, magnesium), potentially boosting crop yields.
MRV Approaches for Engineered Removal
The measurement, reporting, and verification of engineered removals requires methodologies specifically designed for each technology. Verra's VCS Program has approved methodologies for biochar (VM0044) and has methodology development work underway for DACCS and enhanced weathering. The ICVCM's Assessment Framework, when evaluating methodology categories, places particular weight on the robustness of quantification for removal activities, requiring conservative accounting and evidence-based uncertainty factors. Key players developing early MRV approaches include Puro.earth (biochar and wood burial), Isometric (cross-technology registry), and CarbonPlan (independent scientific review of removal projects).
Current Scale and Key Players
As of 2024, total engineered removal credit issuance remains well below one million tonnes per year across all technologies combined. By contrast, the voluntary market as a whole issues several hundred million credits annually, the vast majority from nature-based avoidance projects. The engineered removal sector is growing rapidly from this small base, driven by advance market commitments from early buyers.
A small number of large technology companies have made commitments to purchase engineered removal credits at significant volume as part of their net zero strategies. Stripe Climate launched in 2020 and has committed tens of millions of dollars to early-stage removal purchases. Frontier, a coalition including Stripe, Alphabet, Shopify, McKinsey, and others, announced a $925 million advance market commitment in 2022 to purchase engineered carbon removal between 2022 and 2030. These buyers accept high current costs explicitly to signal demand, build the supplier base, and drive cost reduction through learning curves. The commercial logic is analogous to how solar and wind costs declined through early policy-supported deployment.
Key Takeaways
- 1DACCS, biochar, and enhanced weathering are the three leading engineered carbon removal approaches generating voluntary carbon credits today
- 2DACCS offers the highest permanence (geological storage) but remains the most expensive option at $300 to $1,000+ per tonne and is deployed at tiny scale
- 3Biochar offers moderate permanence (hundreds to thousands of years), greater near-term scalability, and meaningful agricultural co-benefits at costs of $100 to $300 per tonne
- 4Enhanced weathering offers effective permanence but faces the greatest MRV uncertainty because quantifying the geochemical reaction in soils is technically challenging
- 5Early corporate buyers such as Microsoft, Stripe, and the Frontier coalition are purchasing at premium prices to develop the market, analogous to early-adopter demand for solar and wind generation