What does 'measurable climate impact' actually mean in practice?
Measurable means you can directly detect and quantify the CO₂ removed from the atmosphere, not just assume it happened. The strongest direct evidence comes from enhanced weathering: a field study in Virginia used continuous in-soil CO₂ monitors on a soybean plot treated with basalt and recorded a reduction in soil CO₂ flux equivalent to 1.04 tonnes of CO₂ per hectare per year [5]. That removal was most pronounced during the active growing season and after heavy rain, showing that measurement is possible but depends on natural conditions. For biochar, the measurable impact is indirect but still quantifiable: a global meta-analysis found biochar reduces runoff by 25% and soil erosion by 16% on average, which physically protects the carbon stored in soil from being lost [1]. In Mediterranean vineyards, biochar amendments cut soil erosion by up to 65% and increased soil organic carbon by 85% [1]. So while the measurement method differs by approach, both enhanced weathering and biochar provide numbers you can point to.
However, not all methods are equally measurable today. Ocean alkalinity enhancement (OAE) has only one operational deployment—in Halifax Harbour, Canada—which retired credits for 1,800 tonnes of CO₂ by October 2025 [6]. That is measurable at a small scale, but scaling to 1 billion tonnes per year (the threshold for meaningful climate impact) would require a massive expansion, and the study notes that current life-cycle assessments for ocean-based methods cannot yet fully capture impacts on marine environments [2]. So while OAE promises durable removal, its measurability at scale remains unproven.
Which methods are most scalable, and at what cost?
Scalability means going from pilot projects to removing millions or billions of tonnes of CO₂ per year, and the cost per tonne is the make-or-break factor. Biochar combined with phytoremediation (using plants to pull contaminants from soil) shows a remarkable cost advantage: a national-scale simulation across one million hectares of PFAS-contaminated US cropland estimated a median remediation cost of $1,460 per hectare per year—more than ten times cheaper than current thermal destruction or excavation methods [3]. That same system could remove about 11 million tonnes of CO₂ per year, equivalent to 4–6% of the US 2050 carbon removal target, and costs could drop further if carbon removal revenues are valued near the social cost of carbon [3]. For biogas-based hydrogen production, solid carbon from plasma methane pyrolysis applied to agricultural soil costs just 1.3–12.1 euros per tonne of CO₂ equivalent stored, compared to 203–299 euros for geological CO₂ storage, because it avoids capture and shipping [4]. That is a huge cost difference, making soil application of solid carbon far more scalable economically.
But scalability also has environmental limits. Ocean alkalinity enhancement, while theoretically scalable, faces a steep challenge: the study using Nova Scotia as a test case found that moving from 1,000 tonnes of CO₂ removal per year to 1 billion tonnes would require navigating regulatory and geochemical constraints on seawater pH and carbonate saturation [6]. The same study emphasizes that the carbon footprint of delivering alkalinity must be very low for the method to be net-negative [6]. So while biochar and enhanced weathering appear more immediately scalable and cost-effective, ocean-based methods need more development before they can deliver climate impact at scale.
What are the caveats and risks?
Every durable carbon removal method comes with trade-offs that could limit its real-world impact. First, verification is still immature: current life-cycle assessments for ocean-based CO₂ removal have limited scope, often overlook environmental impacts beyond global warming, and cannot fully assess marine environment effects [2]. This means we might be overestimating the net climate benefit of some methods. Second, the choice of how you measure impact matters enormously. A study comparing biochar and BECCS (bioenergy with carbon capture and storage) found that the 'better' system changed depending on whether you measured by heat output, electricity generated, or carbon sequestered [7]. That means two projects could report different climate impacts simply because they used different accounting methods.
Third, some methods have hidden environmental costs. For example, biochar's erosion reduction is a key mechanism for protecting carbon stocks, but if that benefit is not formally included in monitoring and verification frameworks, the permanence of the carbon removal is uncertain [1]. The authors of that study explicitly call for integrating erosion reduction into IPCC methodology reports by 2027 [1]. Finally, the energy background system matters: all biogas-based hydrogen systems can achieve net negative emissions (−6.2 to −13.3 kg CO₂ per kg of hydrogen) only if low-carbon electricity is available [4]. If the grid is fossil-fuel-heavy, the climate benefit shrinks or disappears. So while durable carbon removal can work, its success depends on rigorous, transparent measurement and a clean energy supply.
About These Sources
This answer is built on 7 peer-reviewed studies — published from 2023 to 2026, 6 from 2024 or later, 4 in Q1 journals — selected as the most relevant from 7 studies that passed quality screening, drawn from 56 papers retrieved from a database of over 500 million.
Sources used in this answer
Biochar for durable carbon removal: soil erosion reduction as a key mechanism
A global meta-analysis found biochar reduces runoff by 25% and soil erosion by 16% on average, with Mediterranean vineyard trials showing up to 65% erosion reduction and 85% increase in soil organic carbon, making erosion reduction a core mechanism for carbon durability.
Life cycle assessment of ocean-based carbon dioxide removal approaches: A systematic literature review
A systematic literature review of 20 life-cycle assessments on ocean-based CO₂ removal found that current models cannot fully assess marine environmental impacts and often overlook side effects beyond global warming, urging caution in drawing conclusions.
Integrated thermal and phytoremediation of agricultural soils impacted by PFAS.
A national-scale simulation of PFAS-contaminated US cropland found that combining phytoremediation, biochar, and enhanced weathering could remove ~11 Mt CO₂ per year (4–6% of the US 2050 target) at a median cost of $1,460 per hectare per year—over ten times cheaper than current methods.
Biogas-based hydrogen production and carbon dioxide removal: Techno-economic and climate impact assessment
For biogas-based hydrogen, solid carbon from plasma methane pyrolysis applied to soil costs 1.3–12.1 € per tonne CO₂ equivalent stored, far cheaper than geological storage (203–299 €), and all systems can achieve net negative emissions (−6.2 to −13.3 kgCO₂eq/kgH₂) with low-carbon electricity.
Direct Measurement of Carbon Dioxide Removal Due to Enhanced Weathering
Direct continuous soil CO₂ monitoring on a basalt-amended soybean plot in Virginia measured a CO₂ flux reduction of 1.04 tonnes per hectare per year, most pronounced during the growing season and after rain, providing direct evidence of enhanced weathering rates.
Is the removal of atmospheric CO2 via Ocean Alkalinity Enhancement feasible for climate mitigation?
Using Nova Scotia as a test case, the study found that scaling ocean alkalinity enhancement from 1,000 tonnes of CO₂ removal per year to 1 billion tonnes per year faces major regulatory and geochemical constraints on seawater pH and carbonate saturation, and requires a very low carbon footprint for delivery.
Climate impact of bioenergy with or without carbon dioxide removal: influence of functional unit and parameter variability
Comparing biochar and BECCS systems across four functional units, the study found that the preferable system changed with the chosen metric (heat, electricity, carbon sequestered, or biomass used), and that energy background system assumptions strongly influenced results.