Carbon Removal, Reduction, and Avoidance Credits Explained

Key Takeaways

• CORSIA, the program intended to address international aviation emissions, is currently the world’s largest international compliance carbon market, with approximately 200 million tonnes of eligible emissions units (EEUs) expected to be retired for Phase 1 (2024-2026) compliance. 
• Airlines have already begun retiring significant volumes of credits (clean cookstoves and jurisdictional REDD+) toward their Phase 1 obligations ahead of the January 2028 compliance deadline, signaling that the industry is treating CORSIA as a binding policy mandate.
• In addition to the four already approved, a record 25 carbon standards have applied for eligibility to supply credits under Phase 2 (2027–2035), pointing to a diversifying pipeline of credits.
• Legal enforcement of CORSIA at the national level remains uneven. A handful of jurisdictions, including the EU, Japan, Brazil, New Zealand, and Canada, have established compliance regimes, but it is unclear whether other major aviation markets will implement domestic enforcement requirements by 2027.
• As CORSIA matures alongside other compliance and voluntary frameworks, strategic engagement increasingly depends on understanding where eligibility overlaps and where the most value can be found.

Introduction to CORSIA 

The Carbon Offsetting and Reduction Scheme for International Aviation (CORSIA) was established and is managed by the International Civil Aviation Organization (ICAO)—a specialized agency of the UN—and provides a compliance mechanism to reduce emissions from international aviation that do not fall within a single country’s contributions to the Paris Agreement. For years, CORSIA generated more commentary than activity—airlines weren’t required to offset a single tonne of CO₂ emissions (tCO₂) during the pilot phase (2021–2023). That is changing; in early 2026, Singapore Airlines and Japan Airlines retired significant volumes of carbon credits against their Phase 1 (2024–2026) obligations. Increasing authorization announcements showcase the eligible supply steadily entering the market, and 25 carbon standards have applied for Phase 2 (2027–2035) eligibility. As CORSIA becomes an active compliance market, this article explains how the program works, what is driving the current trends, and what to watch as Phase 1 draws to a close.

Who is Covered by CORSIA Phase 1 and Phase 2?

Because emissions from international aviation are not tied to any individual nation, the International Civil Aviation Organization (ICAO) adopted CORSIA in 2016 to complement the Paris Agreement framework and address these emissions. CORSIA applies to international airlines operating flights registered between participating countries. 

• During Phase 1, offsetting obligations apply only to routes between the 130 countries that have voluntarily opted in. 
• From Phase 2, participation becomes mandatory for all 193 ICAO member states, except for those below the aviation activity threshold or classified as Least Developed Countries, Small Island Developing States,  or Landlocked Developing Countries—although they can voluntarily participate. 

Several major aviation markets, including China, that are not participating in Phase 1 will be required to do so from 2027. 

Offsetting Obligations

Airlines are required to offset the portion of their emissions that exceeds the program’s baseline on routes subject to offsetting. The key input is the annual Sectoral Growth Factor (SGF), which reflects how much annual emissions on covered routes have grown above the baseline (set at 85% of 2019 emissions). In simple terms, each airline's offsetting obligation is calculated by multiplying its CORSIA-regulated emissions by the SGF.

For example, an airline that emits 1 million tCO₂ on covered routes in a year when the SGF is 10%, would have an offsetting obligation of 100,000 tCO₂. An airline that emits 500,000 tCO₂ in the same year would have an obligation of 50,000 tCO₂. The actual 2024 SGF was 15.4%, exceeding the baseline and triggering offsetting obligations for the first time since the COVID-induced reduction in international aviation activity. 

Beginning in 2033, CORSIA’s offsetting formula combined the SGF with an individual component that accounts for each airline's own emissions growth. The individual weighting starts at 15% in 2033 (with an SGF weighting of 85%) and rises to 45% by 2035 (SGF 55%). This progressively ties each airline’s obligations to its own emissions trajectory rather than sector-wide trends alone.

Airlines can also reduce their obligations by reporting the use of CORSIA Eligible Fuel (CEF). In order to qualify as CEF, Sustainable Aviation Fuel (SAF) must meet ICAO’s certification criteria. Offsetting remains a lower-cost option than sustainable fuel procurement, and CORSIA obligations are currently not influential drivers of CEF/SAF uptake compared to supply mandates, such as ReFuelEU Aviation. However, these dynamics will evolve as CORSIA matures and SAF prices continue to fall. 

CORSIA Eligible Carbon Credits: Phase 1 and Phase 2 

To determine the eligibility of carbon crediting standards and credits issued under those standards, ICAO's Technical Advisory Body (TAB) reviews applications and recommends approval to the ICAO Council, subject to restrictions on methodologies, vintages, and more granular project elements. Currently, no standard is approved without exclusions. Credits eligible for use under CORSIA are known as Eligible Emissions Units (EEUs).

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TAB broadly excludes some project types across standards, including large-scale grid-connected renewable energy projects and most project-level REDD+ activities. The exclusions reflect TAB's assessments of baselines, safeguards, and quantification gaps. However, these assessments are not static; ICAO updates its determinations as the evidence base improves. For example, in 2025, TAB approved certain direct air capture (DAC) and biochar pathways for the first time.

A record 25 carbon standards—certifying projects from emissions avoidance to engineered carbon dioxide removal (CDR)—have applied for Phase 2 eligibility in the 2026 assessment cycle, reflecting the scale of the opportunity. The International Air Transportation Association (IATA) projected a demand of 170–236 million EEUs for Phase 1 alone; developers are eager to serve this growing market. Assessment results are expected in October 2026.

Article 6 and Corresponding Adjustments

The biggest bottleneck for eligible supply has been requirements relating to Article 6.2 of the Paris Agreement. CORSIA requires that EEUs carry a host country Letter of Authorization (LoA) confirming that the host country will not count the underlying mitigation toward its Nationally Determined Contribution (NDC). This prevents double counting between CORSIA and national emission inventories, but it also means eligible supply depends on host country authorization, not just meeting methodological requirements. Authorized credits are called Internationally Transferred Mitigation Outcomes (ITMOs). 

To manage the risk that a host country fails to apply the promised corresponding adjustment, projects must secure legally enforceable insurance policies that either provide replacement EEUs or funds to secure them. Several providers have been approved to insure EEUs registered by Gold Standard and Verra. 

Insurance requirements do not apply when the host country has already confirmed the corresponding adjustments in its mandatory Paris Agreement reporting. For example, in March 2026, Madagascar submitted a Biennial Transparency Report (BTR) and annual information report that reflect the application of corresponding adjustments to CORSIA-eligible clean cookstoves credits listed on the Verra registry. According to Verra, BTR accounting is the “highest level of assurance” that ITMOs will not be double counted. 

The Enforcement Challenge

With the authorization bottleneck loosening, market attention is turning to demand. Critically, ICAO has no direct enforcement power; for CORSIA to work, participating countries must pass laws requiring airlines to comply with its requirements. Under CORSIA rules, airlines must retire their Phase 1 EEUs by January 2028, but most are not yet subject to legal sanctions for failing to comply. Where regulations do exist, it remains unclear whether penalties will be stringent enough to matter, or whether they will be enforced at all.

Where legal enforcement of offsetting requirements does exist, it varies by design. For example, in Brazil, failure to comply attracts a penalty of BRL 50/tCO₂ (US$10/tCO₂) and an administrative fine. Meanwhile, the Canadian Aviation Regulations impose administrative fines capped at CAD 25,000 (US$18,000) per infraction, but there is no specific fine for each tCO₂ not offset. However, the absence of a statutory penalty framework does not necessarily mean compliance will not be enforced. For example, New Zealand manages CORSIA participation through an administrative Memorandum of Understanding with Air New Zealand, rather than a dedicated legal regime (though this will change from 2027). Similar informal arrangements may be more common than the legislative record suggests.

According to 2024 data, US-based airlines account for around 15% of CORSIA-covered emissions. However, there is no indication that the federal government will implement CORSIA penalties under the current administration. It is unclear how non-compliance by a major economy will impact market confidence or enforcement by other governments, but uncertainty about US participation in multilateral climate programs is not new. 

What to Watch

• Global enforcement: As Phase 2 approaches, the prospects for CORSIA enforcement in key aviation markets will become clearer. China, which has not participated in the voluntary phase and has raised objections to CORSIA’s design on grounds of fairness, presents an uncertain picture for domestic transposition. The UK government has consulted on a draft legal amendment that would penalise non-compliance for UK airlines at £100/tCO₂ (US$130/tCO₂), but has not published updates since February 2025. 

• EU Emissions Trading Scheme (ETS) update: In July 2026, the European Commission proposed to expand the ETS to address emissions from flights departing the European Economic Area (EEA) to destinations within 5,000 km. To avoid double charging emissions also regulated under CORSIA, the Commission proposed to deduct CORSIA costs from airlines’ ETS obligations. While the Commission flagged concern about the stringency of CORSIA eligibility criteria, the proposed rules would not impact demand for Phase 1 EEUs. Still, it is worth watching for future restrictions on which credits EU airlines can use for CORSIA, whether in Phase 1 or 2. 

• Supply financing vehicles: The LEAF Coalition—which purchases credits verified under the CORSIA-eligible ART TREES standard—has signed over US$1.5 billion worth of purchase agreements with host countries. The Forest Carbon Partnership Facility (FCPF) and BioCarbon Fund Initiative for Sustainable Forest Landscapes (ISFL), both CORSIA-approved for Phase 1 in early 2026, have built jurisdictional REDD+ pipelines across dozens of countries. As projects mature, it will be worth watching how issued credits are marketed across compliance and voluntary channels.

Strategic Considerations

Avoided emissions credits dominate early CORSIA supply—and consequently demand—principally from avoided deforestation and clean cooking projects. As there is no regulatory premium for higher-cost CDR, this is not surprising. But as the market matures and supply diversifies, airlines are likely to begin differentiating on geography, co-benefits, and environmental performance. Corporate interest is growing in projects that abate non-CO₂ superpollutants, due to their outsized near-term climate impact. As many of these projects are based in countries actively issuing LoAs, this may soon translate into a supply of superpollutant EEUs. Some airlines are reluctant to procure aggressively now, anticipating the upcoming availability of higher-quality credits.

Credits that hold value across multiple frameworks could carry a strategic premium. A credit also eligible for Singapore's carbon tax, Switzerland's CO₂ Act, or SBTi claims may offer more durable demand than one tied to a single use case. While specific requirements vary significantly, we are beginning to see convergence around broad principles, such as the need for Article 6 authorization for credits used in compliance markets tied to NDCs. Procurement and project development strategies could begin to favour credits that confer wider market optionality. 

The precondition for all of this is policy stability. Longer-term offtake agreements, portfolio strategies, and meaningful investment in project development depend on confidence that CORSIA rules will hold. Efforts to soften emissions regulations in response to energy price shocks are now testing this confidence, from proposed changes to soften the EU ETS to Singapore's deferral of its SAF levy. Tracking price signals, enforcement developments, and authorization trends will be essential for identifying firm demand and where the opportunities lie.

Conclusion

Effective engagement with CORSIA requires more than developing quality projects. It means understanding methodological eligibility requirements, navigating Article 6.2 authorization processes, tracking a network of overlapping national regulations, and staying ahead of a rapidly evolving market landscape. Relae (formerly Carbon Direct) brings together policy, market, and scientific expertise to help clients on all of these fronts.

Frequently Asked Questions

Which carbon credits can airlines use for CORSIA compliance?

CORSIA Eligible Emissions Units (EEUs) must be issued by crediting standards approved by ICAO's Technical Advisory Body. Every approved standard currently has exclusions on certain methodologies and project types, so eligibility is granular rather than blanket. EEUs must also have a host country Letter of Authorization confirming a corresponding adjustment under Article 6.2 of the Paris Agreement.

Is CORSIA actually enforceable?

In all phases, CORSIA obligations are legally binding when a country writes them into domestic law. Countries participating in Phase 1 are expected to do this, but not all have, and enforcement remains uncertain in several key aviation markets (e.g., the United States). Even where enforcement exists, penalty regimes vary significantly in stringency and design. 

How does the July 2026 EU ETS proposal affect CORSIA?

The European Commission's proposal to expand ETS coverage to more flights does not reduce demand for CORSIA credits. While it would lead to overlap between flights covered under CORSIA and the ETS, as proposed, airlines could have CORSIA costs effectively deducted from their ETS allowance obligations. However, the Commission has signalled continued scrutiny of CORSIA's effectiveness, which could shape future limits on the credits EU airlines are allowed to use.

Key Takeaways

• The Greenhouse Gas (GHG) Protocol’s Actions and Market Instruments (AMI) proposal would formally recognize market-based instruments across emissions scopes for the first time, within Statement 2.
• The proposal substantially expands opportunities for companies to report lower emissions from purchasing market-based instruments for low-carbon fuels (LCF), such as book-and-claim environmental attribute certificates (EACs) for sustainable aviation fuel (SAF) and renewable natural gas (RNG).
• Companies purchasing or already holding market-based instruments for SAF, RNG, and other low-carbon fuels should begin mapping those instruments to the AMI’s proposed reporting structure now, before the standard is finalized.

The Current Reporting Problem With Low-Carbon Fuels

A low-carbon fuel is defined as a fuel whose lifecycle greenhouse gas (GHG) emissions are lower than those of a relevant, use-case-specific fossil fuel baseline. Most LCFs used today are created from biogenic sources like agricultural residues, used cooking oil, or landfill gas, and their use often results in reduced GHG emissions due to the fact that the CO₂ they release during use is biogenic and not fossil.

Consumers of LCFs face a reportability problem. Many buyers track LCF purchases through market-based instruments such as book-and-claim and mass balance EACs. These instruments unbundle environmental claims from physical molecules to facilitate investment in LCFs when direct provision of the molecule is infeasible. However, the current GHG Protocol only permits the use of market-based tools for electricity-based emissions. 

Currently, companies cannot recognize the emissions benefit of an LCF purchase unless they physically receive and combust the fuel. Given that sustainable aviation fuel, renewable natural gas, and other LCFs typically flow into shared pipeline and distribution infrastructure rather than being delivered to buyers as distinct physical molecules, direct delivery is often not achievable in practice. This limitation constrains demand growth in the voluntary market.

The value of LCFs to voluntary buyers is the sustainability claim attached to the molecule, not the molecule itself. Without a credible reporting framework for market-based instruments, buyers struggle to justify LCF procurement. Producers, in turn, may face a market where buyer reluctance limits commercial opportunities for scale-up. Non-reportability inhibits one of the few available levers to incentivize voluntary investment in climate solutions.

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How Does the AMI Proposal Address Non-Reportability?

The Actions and Market Instruments (AMI) proposal introduces a four-statement reporting framework, with all four statements relevant to elements of LCFs. Together, these statements give organizations across the LCF value chain a standardized way to account for LCF procurements in their sustainability reports, whether those procurements are physical or through market-based instruments.

• Statement 1 covers the physical GHG inventory and is the traditional GHG Protocol inventory as it exists today. Companies that physically receive and combust LCFs recognize the lower GHG emissions here.
• Statement 2 proposes a market-based GHG inventory for all emissions scopes. For the first time, this will allow companies to report the carbon intensities of book-and-claim and mass balance EACs for LCFs within a GHG Protocol-aligned inventory. This is a shift from the current framework, which relegates market-based instruments for LCFs and other products, along with carbon credits, to a supplemental section outside of the core emissions inventory. By bringing these purchases into the inventory itself, Statement 2 puts market-based LCF claims on a more equal footing with the emissions companies already report.
• Statement 3 covers beyond value chain interventions as well as GHG emissions reductions/avoidance reported relative to a baseline counterfactual (e.g., the domain of carbon credits). Avoided methane emissions upstream from RNG use is one LCF-relevant example that might live here. 
• Statement 4 captures non-GHG indicators and could include things such as the quantity of an LCF procured or produced. This rounds out the story an organization tells about how LCFs fit within their operations and value chain.

How the AMI Proposal Applies to Specific Fuel Types

Two important LCFs are impacted by the current GHG Protocol guidance and offer an example of the significance of the AMI proposal. 

1. SAF is a biobased or synthetic drop-in substitute for conventional jet fuel. 
2. RNG, generally produced from organic wastes, is a drop-in substitute for conventional natural gas. 

Both are typically delivered through existing infrastructure. SAF is typically blended into a common fuel supply. RNG is often injected into the broader gas network. The buyer typically does not have a direct physical link to the molecule’s consumption, but rather an indirect claim on molecules injected into a common system.

Current GHG Protocol guidance disallows using and reporting market-based LCF emissions factors within corporate scope inventories. If finalized as proposed, the AMI proposal’s Statement 2 would open the aperture for companies to report the lower emissions from purchasing SAF or RNG market-based instruments. Here’s how that might work.

SAF: Reporting Across the Value Chain

When an airline physically receives and burns SAF in its aircraft, the lower combustion emissions are reflected in its Statement 1, scope 1 emissions inventory. This is reported the same way as conventional jet fuel.

The picture changes when airlines use market-based instruments. Many airlines purchase scope 1 SAF certificates that are decoupled from the physical fuel and tracked via book-and-claim or mass balance EACs. Under the AMI proposal, the lower emissions factors of these instrument purchases would be reflected in an airline’s Statement 2, scope 1 emissions inventory.

Corporate buyers face a parallel situation. Companies that purchase scope 3 EACs from SAF suppliers or through airline programs to address employee travel would reflect these lower emissions factors in their Statement 2, scope 3, category 6 emissions. Legitimate Statement 2 claims provide additional incentive for corporate climate action to address travel emissions.

RNG: Reporting Across the Value Chain

As with SAF, an organization that physically receives and combusts RNG can reflect the lower emissions in its Statement 1, scope 1 emissions inventory. 

More often, RNG is injected into the shared gas network and claimed by users through book-and-claim or mass balance certificates rather than physical delivery. Statement 2 introduces a reporting opportunity for RNG EAC purchasers to report lower scope 1 emissions. Lower emissions from RNG consumption could also be passed through as reduced scope 3 emissions for consumers of products associated with RNG consumption.

RNG carries an additional nuance. Some RNG projects may also claim to have avoided methane emissions (e.g., dairy manure methane emissions avoided by way of diversion to RNG). This type of methane avoidance attribute, typically tracked in the realm of carbon credits, would have a home within Statement 3, subject to additional disclosure requirements. Companies evaluating RNG certificates should understand which claims belong in which statement.

Application to Other LCFs and Market-Based Instruments

While SAF and RNG are two of the more prominent examples, the AMI proposal is relevant to all LCFs including renewable diesel and marine fuels. The emissions associated with market-based instrument purchases for these fuels would belong in Statement 2.

The proposed framework may also cover market-based instruments aimed at lower emissions conventional fuels, e.g., natural gas production that has taken measures to reduce fugitive methane emissions far below industry averages. Companies purchasing certificates of this type would theoretically report the reduced upstream natural gas emissions rates in Statement 2, scope 3, category 3.

Three Steps Companies Can Take Now to Prepare 

The AMI proposal is still in development. A public request for information on the initial white paper recently closed. A draft standard is in development, with a formal public consultation planned in Q3 2027. Companies that act early will be better positioned when the standard takes effect. Steps will vary depending on where in the fuels value chain an organization sits, but could include:

1. Map exposure. Identify where the organization sits in the LCFs value chain and what fuels it is currently procuring and consuming. This will help to better understand where the largest fuels-related emissions hot spots are in the current GHG inventory.
2. Record current instruments and assess quality. Catalog the instruments the company is currently producing or procuring. Understand where these instruments may sit within the AMI’s proposed four-statement framework and assess their quality against emerging eligibility criteria.
3. Engage with the process. The GHG Protocol is still defining eligibility and quality criteria for LCF instruments. Organizations can monitor interim developments from the AMI Technical Working Group. Companies with active programs have a real stake in how those criteria are written.

Frequently Asked Questions

How would the AMI proposal change the way low-carbon fuels get reported?

The core shift is the introduction of Statement 2, a new market-based GHG inventory. For the first time, companies would be able to report the lower emissions from purchasing market-based LCF instruments, including book-and-claim and mass balance EACs, within a GHG Protocol-aligned inventory. This applies to all LCFs, including SAF and RNG, and can be reflected in scope 1 or scope 3 inventories depending on where the organization sits in the LCF value chain.

Is the AMI proposal finalized, and what should companies do now?

No, the proposal is still in development. A public request for information on the initial white paper recently closed. A draft standard is in development, with a formal public consultation planned in Q3 2027. In the meantime, companies holding SAF, RNG, or other LCF instruments should map their exposure across the value chain, catalog existing instruments against the proposed four-statement framework, assess quality against emerging eligibility criteria, and monitor GHG Protocol AMI updates.

‍What should a company look for when evaluating a SAF or RNG certificate?

Buyers should confirm which claim a certificate actually conveys and where it would sit in the AMI’s proposed framework. For example, a lower-emissions carbon intensity claim belongs in Statement 2 whereas an avoided-methane claim falls under Statement 3. Buyers should also assess the instrument against the AMI's emerging eligibility and quality criteria to reduce the risk that a purchase won't qualify under the final standard. Relae (formerly Carbon Direct) has published a comprehensive set of criteria for high-quality low-carbon fuels that buyers can use as a starting point, until AMI’s eligibility and quality criteria are finalized.

Key Takeaways

• Near-term role for carbon removals: The revised Science-Based Targets Initiative (SBTi) Corporate Net Zero Standard (CNZS) v2.0 proposes requiring or recognizing interim carbon dioxide removal (CDR) targets before companies reach net zero. This shift aims to scale up the CDR industry in line with climate science and ensure companies address residual emissions earlier rather than deferring action until their net-zero target year.‍
• Revised target frameworks and increased accountability: The draft standard introduces key changes, including separate targets for scope 1 and 2 emissions, enhanced scope 3 target-setting guidelines, and the recognition of market-based mechanisms for indirect mitigation of scope 3 emissions. It also requires companies to publicly disclose transition plans and assess implementation progress against targets.‍
• Potential for stronger action on removals: While the proposed updates are a step forward, the final standard should make near-term CDR targets mandatory rather than optional and should expand to include those for projected scope 3 residual emissions, not just scope 1. Without these stronger mandates, demand for early-stage CDR investments may remain limited, potentially slowing progress toward net-zero goals.‍
• Companies should prepare now: Businesses should start integrating CDR into their climate strategies now so they are equipped to navigate procurement of high-quality removal credits in the voluntary carbon market. Preparing ahead of the final CNZS v2.0 release will help companies align with science-based decarbonization pathways and demonstrate climate leadership.

What the Latest SBTi Update Means for Corporate Climate Action

On 18 March, 2025, the Science Based Targets Initiative (SBTi) released its draft Corporate Net Zero Standard (CNZS) v2.0, significantly updating its framework for corporate net-zero target setting for the first time since 2021. The draft proposes new approaches for companies to support carbon dioxide removal (CDR) in the transition to net-zero emissions, a move that could help scale the nascent CDR industry.

This release is part of a broader revision, with key proposed updates including:

• Requiring public disclosure of transition plans after companies set targets.
• Separating targets for scope 1 and 2 emissions.
• Enhancing the scope 3 target-setting framework using an impact-based prioritization process.
• Recognizing indirect mitigation (e.g., book-and-claim commodity certificates) for hard-to-trace scope 3 emissions.
• Defining a role for emissions removals in the transition to net zero.
• Providing options to recognize company leadership in beyond value chain mitigation (BVCM).
• Assessing corporate progress against targets to bolster accountability.

While formal recognition of removals in the transition to net zero is a positive step, stronger incentives will be needed in the final CNZS to ensure companies take meaningful early action on CDR. The draft standard is open for the first of two public consultations until 1 June, 2025, with a finalized version expected to launch in 2026.

What Remains the Same From the Current Standard?

SBTi’s framework remains focused on three core requirements for companies:

• Reducing emissions year-on-year to reach an approved science-based target (SBT) by 2050 or earlier.
• Investing in beyond value chain mitigation (BVCM) in the transition to net zero to support near-term global decarbonization efforts.
• Neutralizing remaining emissions from the net-zero year (achieved after at least 90% emissions reductions) and onwards with high-quality, permanent carbon removal.

Carbon credits representing emissions reductions and removals remain ineligible for meeting reduction targets within a company’s value chain (SBTs).

What’s New in CNZS v2.0 for Carbon Removals?

Until now, the SBTi encouraged companies to invest in CDR through mechanisms such as BVCM, but has not proposed requiring removal before their target net-zero year. As a result, organizations had little clarity or incentive to invest in CDR ahead of their net-zero target date, dampening near-term demand for carbon removal and delaying the industry's growth.

The new draft changes this by proposing three options for the V2.0 Standard that address the impact of residual emissions during the transition to net zero:

• Option 1 (requirement): Companies are required to set near- and long-term removal targets, including interim CDR milestones,¹ to address projected residual emissions.
• Option 2 (optional with recognition): Companies can set and receive recognition for removal targets to address projected residual emissions.
• Option 3 (flexibility of mechanism): Companies have the flexibility to address expected residual emissions either entirely through additional emissions reductions within their value chain, entirely through removals, or via a combination of both.

Notably, all three approaches apply only to residual scope 1 emissions.

On top of these approaches, SBTi has suggested two options for the minimum durability threshold of CDR purchases in their draft standard. Removals will either need to follow a ‘like for like’ approach,² where CDR storage must match the atmospheric lifetime of residual emissions, or a gradual transition approach, where carbon storage durability increases over time.

Why Mandating CDR Matters

Mandating near-term CDR reinforces the need for immediate climate action, ensuring that carbon removals complement emissions reductions rather than being deferred until the net-zero target year. The urgency of early CDR investment is clear:

• Limiting global warming to well below 2°C above pre-industrial levels requires removing billions of tonnes (gigatonnes) of carbon dioxide annually by mid-century.
• The CDR industry is in its early stages and requires sustained investment today to scale in time.
• If companies wait until their net-zero year to purchase CDR, the supply of high-quality removal credits is unlikely to be sufficient.

As part of the proposed removal targets in Option 1 (above), the CNZS v2.0 would require companies to gradually increase CDR purchases over time, ramping up to 100% of a company’s projected residual scope 1 emissions in the net-zero target year (<10% of baseline year emissions).³

What More Can Be Done?

The potential introduction of required removal targets would be a significant and welcomed step, reinforcing the importance of near-term CDR investment to support industry maturation and climate goals. However, mandatory near-term CDR targets represent only one of three potential pathways for the V2.0 standard. Moreover, neutralization of residual emissions on the path to net zero is only proposed for scope 1 emissions. While the SBTi provides clear rationale for this,⁴ SBTi should not let the complexity of projecting scope 3 emissions be a barrier to climate change mitigation. Scope 3 emissions represent the majority of emissions from SBTi-aligned companies. Furthermore, companies with high scope 3 emissions typically have a higher ability to pay compared to their industrial counterparts with high scope 1 emissions.

If interim removal targets are made optional, and scope 3 emissions remain excluded, the demand signal for near-term CDR will be limited. The public consultation (and advice of Expert Working Groups that SBTi is convening) will be essential in determining which proposed guidance matures into the final standard; consultation feedback can be provided here prior to June 1, 2025.

How Businesses Can Prepare for CNZS v2.0

Companies aligning with SBTi’s evolving guidance should begin to integrate CDR into their climate strategy now. This means:

• Understanding their residual emissions forecast and planning early investments.
• Developing a company-specific climate strategy to incorporate CDR into their sustainability roadmaps in tandem with plans to reduce value-chain emissions.
• Engaging early in the voluntary carbon market to implement these strategies and develop procurement processes to support high-quality removal projects.
• Ensuring credibility by selecting removal projects aligned with scientific best practices.

Conclusion: A Step in the Right Direction, But More Certainty Is Needed

The CNZS v2.0 draft represents a critical turning point for corporate climate action with options to formalize the role of early CDR investment through interim removal targets for signatories. However, SBTi must take a stronger stance in the final version of the revised standard by choosing to adopt requirements for near-term neutralization (rather than leaving this optional), including projected residual scope 3 emissions in near-term CDR targets, aligning durability requirements with climate science, and defining removal quality standards to ensure these efforts drive meaningful climate impact. Organizations seeking to align with this guidance should prepare by developing a comprehensive climate strategy that accounts for science-based decarbonization pathways and recognizes the role of early-stage investment in CDR solutions.

Key Takeaways

• Biochar is the fastest-growing durable carbon removal method in the voluntary carbon market, accounting for 93% of all durable removal volumes contracted in the first quarter of 2026 alone.
• Scientists still have an incomplete understanding of biochar durability in soils, and there is some uncertainty regarding biochar's ultimate climate impact.
• Biochar carbon removal crediting must reflect conservative, science-based estimates of durability to safeguard against overcrediting.
• Relae’s (formerly Carbon Direct) detailed analysis highlights the key scientific issues affecting biochar soil durability to better inform credible carbon removal.

Biochar Is Gaining Momentum in Carbon Removal

Biochar has entered the spotlight as a scalable climate solution. It is being adopted for its carbon storage potential and agricultural co-benefits. Commercial uptake of biochar has expanded dramatically since 2019 and continues to accelerate, due to the voluntary carbon market. In the first quarter of 2026 alone, 2.3 million tonnes of total durable carbon dioxide removal (CDR) have been contracted, with biochar accounting for 93% of that volume, driven substantially by a single Microsoft-Liferaft deal for one million tonnes. Established and emerging CDR registries have developed biochar protocols and are certifying biochar carbon credits.

In addition to CDR, biochar can bring numerous co-benefits when it is applied to soil. These benefits include improved water and nutrient retention, pH adjustment, and pollution remediation. In agricultural and forage systems, improvements in soil quality following biochar application can lead to enhanced plant growth. Biochar also promotes rural economic development and can be a part of a circular economy that stewards its resources judiciously.

As purchases of biochar credits grow, a central question has not yet been fully answered: How durable is biochar carbon storage, and what are the best methods to assess this? This blog explores the science of biochar durability, also known as permanence, and how credible science can inform the carbon removal market.

What Is Biochar and How Is It Made?

Biomass heated in an oxygen-deficient environment produces biochar, a solid, carbon-rich material. In the near or full absence of oxygen, that process is called pyrolysis. Biochar can be produced from various kinds of biomass, including agricultural or forest residues, construction debris, organic waste, seaweed, and more. Biochar is also gaining attention for its potential as an ingredient within engineered materials, such as cement.

Biochar achieves CDR by durably storing carbon dioxide (CO₂) that was removed from the atmosphere via photosynthesis and plant growth. Carbon stored in living plants will stay stored during the plant’s lifespan. Once it dies, a plant’s biomass decomposes, cycling most of the stored carbon back into the atmosphere. By introducing technological interventions into this carbon cycle, such as using pyrolysis to produce biochar, the carbon that nature captured via photosynthesis can be stored longer than it would have been otherwise. Pyrolysis converts some of the carbon from biomass into chemical compounds with a higher resistance to environmental degradation. How much carbon converts to these more recalcitrant forms depends on the type of biomass used and the heating conditions, including temperature and time. After biochar is produced, the highly recalcitrant fraction of carbon can be stored for long periods of time, as long as the biochar does not combust.

Common methods of biochar production include:

• Industrial pyrolysis systems: Modern pyrolysis systems heat biomass in a low or no-oxygen environment, creating biochar and gaseous compounds known as synthesis gas. Synthesis gas is typically combusted in a controlled system to provide heat for the system. Low air emissions can be achieved for these systems, which is important to maximizing the environmental benefits of biochar production.
• Modular pyrolysis systems: Smaller, transportable pyrolysis units serve farm-scale or decentralized production. Unlike open kilns, these systems retain temperature control and sensor-based monitoring similar to industrial pyrolysis, and some use autothermal designs that recycle non-condensable gases for heat rather than relying on an external energy source. This makes them a middle option for smaller producers seeking more process control and emissions oversight than open-kiln methods allow, without the capital cost of a large, fixed industrial plant.
• Kon-Tiki kilns: This is a less complex and, therefore, more widely deployable method of biochar production. However, air pollution from this method is likely to be higher than from industrial pyrolysis and can be significant enough to undermine the carbon benefits of biochar production. In Kon-Tiki kilns, biomass is burned in an open-top, conical kiln. Biomass is continually added to the process and finally quenched with water.
• Trench mounds: Trench production is one of the simplest forms of biochar production and is used often by smallholder farmers. Biomass is buried in a trench, covered, and combusted to produce biochar. Air pollution from tench mound production is likely to be significant and even higher than from Kon-Tiki kilns.  

How Biochar Durability Is Quantified

Durability, or permanence, is a key metric used to quantify carbon removal from biochar; it describes the amount of carbon that remains stored in biochar for 100 years or more after it is applied to soil.

Quantifying durability relies on experiments and models to project permanence. There are a number of established and emerging approaches to evaluating biochar recalcitrance to chemical decomposition and storage permanence. Most of these are based on short-term incubation experiments in laboratories, where scientists seal biochar in a container with other organic matter and measure carbon changes over a period of weeks to months. These approaches typically treat biochar as having multiple pools of carbon that differ in their ability to resist decomposition.

While these experiments offer insights into decomposition dynamics, they have two significant limitations. First, these approaches do not measure biochar decomposition and durability in the real world (e.g., after biochar is applied to agricultural soils), meaning they do not capture the effects of actual soil and other environmental factors. Research has shown that different soils can strongly influence decomposition rates. Second, the short duration of these experiments means there is significant potential for error in extrapolating the results to time periods of centuries and beyond.

Why Quantifying Durability Accurately Is Critical

Biochar CDR certification methodologies use a range of approaches to quantify permanence. Most commonly these are based on biochar’s chemical composition, namely the hydrogen-to-organic-carbon ratio (H/Corg). Models that correlate H/Corg with projected permanence, from decay models applied to biochar decomposition experiments, are used to calculate a permanence factor. Other quantification methods include measuring the random reflectance of biochar to estimate the amount of highly recalcitrant carbon in biochar.

The risks of inaccurate durability estimates for carbon removal buyers are significant. Overestimating durability can lead to ineffective CDR spending, reputational risk from overstated climate impact, and delayed progress on net-zero and compliance goals. Buyers need to understand and engage with durability estimation methods to ensure they are procuring high-quality biochar credits.

Why Underground Coal Isn't a Valid Durability Comparison

Recent research proposes a new approach to quantifying biochar durability. The authors assert that random reflectance—a parameter developed in the field of organic petrology (the study of organic matter in rocks)—can be used to characterize durability. Using a component of coal (known as "inertinite") as an analogue for biochar, they propose a benchmark random reflectance of 2% as an "inertinite benchmark" for biochar. They argue that the entire fraction of biochar that meets that benchmark will degrade over approximately 100 million years. This is a large departure from other estimates of biochar durability, and we believe this to be at odds with more conservative models.

There are several reasons to doubt this method. The coal-biochar analogy is inadequate because coal buried underground is not exposed to the same degradation conditions as biochar in soil. Underground coal has lower exposure to oxygen, water, and ultraviolet light, all of which are plentiful in surface soils. Each of these is associated with well-understood decay mechanisms. Furthermore, the process of geologic burial dramatically changes the physical structure of organic matter. Additionally, the permanence factor that the reflectance method allows (i.e., up to 100% carbon retention over extremely long time scales) contradicts experimental evidence that some carbon loss will occur over years or decades.

Subsequent research from the same authors has since recommended pairing random reflectance analysis with H/Corg rather than relying on either alone. This is a refinement of the original approach meant to support more reliable quantification of biochar durability and avoid overcrediting CDR from biochar.

Where Biochar Science Is Headed Next

Biochar stability science will keep advancing. Research into biochar degradation pathways in soil will continue to refine permanence models and inform the relevance of coal analogues. Long-term field measurement studies, with a high density of in-soil sampling to quantify persistence over time, should be a near-term priority that biochar CDR projects can contribute to. Additional studies should investigate pathways to increase durability and rigorously develop modern isotopic, spectroscopic, and chemical techniques for enabling robust assessments of biochar durability.

Crediting Biochar Means Getting Durability Right

Biochar is a promising solution for both soil health and climate impact. Nevertheless, carbon removal certification protocols will need updating as the science of biochar durability evolves. Relae cautions against excessively generous claims of biochar CDR durability, in both permanence factor and timeframe, given the important uncertainties outlined above.

Protocols should not outpace science in terms of permanence assessment, and crediting should stay rooted in an understanding of what drives biochar permanence that is backed by science, including production conditions (e.g., high temperatures) and biochar composition (i.e., low H/Corg). At a time when investment in biochar is booming and the science of biochar is evolving, the voluntary carbon market must keep pace.

Frequently Asked Questions

Is biochar carbon negative?
Biochar can provide net carbon dioxide removal when the amount of carbon durably stored exceeds the emissions associated with producing, transporting, and using the biochar. Plants capture CO₂ from the atmosphere through photosynthesis and store that carbon in biomass. Left alone, most of that carbon returns to the atmosphere within years to decades as the biomass decomposes or burns. Converting biomass to biochar stabilizes a portion of that carbon into forms that can stay stored for centuries or longer, delivering a net removal once those production emissions are accounted for..

Is biochar considered an engineered, hybrid, or nature-based carbon removal solution? Biochar is considered a hybrid carbon removal solution because it combines natural carbon uptake through photosynthesis with a technological intervention. Biomass first removes and stores atmospheric carbon as it grows, and pyrolysis then converts a portion of that biomass carbon into more stable forms that keep carbon from returning to the atmosphere for longer periods.

How do carbon removal registries currently estimate biochar's durability? Registries use decay models based on biochar's ratio of hydrogen to organic carbon (H/C_org ratio), which correlates with how resistant the biochar is to microbial breakdown, combined with environmental factors where biochar is applied, like soil temperature, moisture, and application context, to estimate what fraction of the carbon remains stable (i.e., not converted back to CO₂) over a defined timeframe. Methodologies typically also require third-party lab testing of feedstock and pyrolysis conditions to substantiate durability claims. Some registries and methodologies also incorporate random reflectance as part of durability quantification, although the scientific basis for using reflectance to estimate long-term biochar durability remains under debate.

How long does biochar actually store carbon? 

Biochar does not have a single storage lifetime. Some of its carbon decomposes relatively quickly, while more stable fractions can remain stored for centuries or longer, depending on the biochar’s properties and the conditions where it is stored. Because these estimates rely on models rather than direct observation, carbon crediting has to balance two risks: crediting more durability than actually exists, and being so conservative it undercredits real removals and threatens project viability.

Key Takeaways

• Concrete and steel contribute 13% of global CO2 emissions. These materials represent substantial scope 3 emissions for companies that are building new infrastructure, including data centers.
• Low-emission alternatives and production technologies are nascent and not yet widely deployed.
• Environmental attribute certificates (EACs) are an innovative solution that can stimulate manufacturing of sustainable commodity materials and overcome market barriers to direct physical procurement.
• Relae (formerly Carbon Direct) and Microsoft developed criteria to guide Microsoft’s high-quality EAC procurement, accelerate decarbonization, and catalyze market expansion for decarbonized materials across supply chains. 
• These criteria cover seven key areas: qualifying conditions, social harms and benefits, environmental harms and benefits, additionality and baselines, catalytic impact, verifiability, and leakage.

A Vision for Low-Carbon Concrete and Steel

Concrete and steel are essential for modern construction, but are carbon-intensive. Combined, they account for approximately 13% of global carbon dioxide (CO2) emissions. ​As the demand for data centers and infrastructure development continues to grow, so do the embodied emissions from concrete and steel. There is an urgent need to decarbonize these sectors so that construction of new facilities remains consistent with climate commitments.​

However, the markets for low-carbon concrete and steel are still in their infancy. Market barriers such as low production volumes, contracting complexity, geographic concentration of supplies, and long-distance transportation limit buyers’ ability to directly procure low-carbon building materials. As buyers and sellers overcome these barriers, the market for low-carbon building materials will mature, giving producers the confidence to invest in new manufacturing facilities and giving buyers access to decarbonized materials directly in their supply chain.

Environmental attribute certificates (EACs) represent the sustainability attributes of a material commodity, unbundled and transacted separately from the physical commodity. EACs are transacted at a price that reflects the additional cost of sustainably producing the material. While barriers to direct procurement of low-carbon materials persist, EACs can serve as a powerful tool to signal demand for low-carbon concrete and steel, catalyze market expansion, and accelerate decarbonization.

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Microsoft’s Approach to Emissions from Building Materials ​

To address embodied emissions and help achieve its ambitious 2030 carbon-negative goal, Microsoft aims to deeply reduce its scope 3 emissions, including from building materials, by more than half compared to a 2020 baseline. In 2023, scope 3 emissions made up over 96% of Microsoft’s total greenhouse gas footprint. To meet its 2030 target, Microsoft is focusing on addressing supply-chain emissions related to data center construction and the embodied carbon of the commodity materials used to build them. Microsoft is already working to decarbonize its building material supply chain using the following three tactics:

• Reduce the need for concrete and steel through innovative building design or the use of lower-carbon alternatives, such as sustainably sourced mass timber.
• Directly engage with suppliers to procure low-carbon alternatives to conventional concrete and steel.‍
• Invest in and help pilot new low-carbon production pathways.

EACs represent a fourth approach to decarbonization that can both reduce emissions and catalyze industry growth. Taken together with Microsoft’s design, procurement, and investment tactics, these actions underpin a comprehensive strategy for long-term decarbonization of the built environment.

High-Quality EACs Support Decarbonization

EACs offer a virtual, market-driven solution to decarbonize the built environment by allowing companies to support low-carbon material production when physical supply chain barriers impede direct procurement. ​EACs for concrete and steel function similarly to other market-based decarbonization mechanisms such as sustainable aviation fuel (SAF) certificates in the aviation sector and renewable energy certificates (RECs) in the energy sector.

These mechanisms work by decoupling the environmental benefits of low-carbon production from the physical materials themselves. EACs allow companies to claim the environmental benefits of physical low-carbon concrete or steel production, even if they do not directly or physically source those materials. EACs provide a verified claim that a purchaser has procured a product with lower emissions, enabling companies to meet climate targets and demonstrate their commitment to sustainability.

Due to their virtual nature, which limits physical oversight of the supply chain, EACs also exacerbate risks such as double counting, greenwashing, fraud, social harms, and environmental harms. Therefore, adhering to transparent, rigorous criteria is foundational to mitigating these risks.

New Criteria to Guide Procurement of High-Quality EACs

To support the integrity and effectiveness of EACs in these sectors, Relae and Microsoft have co-developed Criteria for High-Quality Environmental Attribute Certificates in the Concrete and Steel Sectors. These robust criteria serve as a public resource for companies seeking credible and impactful EAC procurement pathways for low-carbon concrete and steel.

The criteria address seven key areas:

1. Qualifying conditions: EACs must complement, not displace, direct procurement and represent significant emissions reductions. ​
2. Social harms and benefits: EACs must not further social harms and should promote community engagement and social benefits such as meaningful workforce development opportunities and community climate resilience.
3. Environmental harms and benefits: EACs must not perpetuate environmental harm, but instead should mitigate risks and, ideally, provide additional environmental benefits.
4. Additionality and baselines: Projects funded by EACs must drive real, meaningful emissions reductions ​in terms of financial, regulatory, and common practice additionality.
5. Catalytic impact: Prioritize transformative technologies with sector-wide decarbonization potential. ​
6. Verifiability: Set a high bar for documentation, transparency, and independent auditing. ​
7. Leakage: Address risks of emissions displacement to support net-positive outcomes. ​

These criteria provide a robust framework for companies and stakeholders to evaluate and implement commodity EACs effectively, supporting impactful change. ​They balance rigor with flexibility and acknowledge the nascent state of the markets for low-carbon concrete and steel. Sectoral decarbonization will advance over time, shifting the thresholds for what is considered significant, additional, and catalytic in EAC transactions. This science-driven framework can adapt to advancements in policy, industry practices, and technological readiness, ensuring EACs remain a relevant and impactful tool for years to come. ​

Sector-Specific Insights

In addition to providing overarching criteria and guidance on procuring EACs for the built environment, Criteria for High-Quality Environmental Attribute Certificates in the Concrete and Steel Sectors also provides insights into the unique challenges and opportunities of decarbonizing concrete and steel.

Decarbonizing Concrete

EACs can support innovative decarbonization strategies for concrete, such as producing alternatives to clinker and cement, adopting supplementary cementitious materials (SCMs), and reducing emissions from cement kilns through electrification or integrating carbon capture and storage (CCS). ​Performance-based standards and evolving building codes may also unlock new opportunities for low-carbon concrete. ​

The Global Cement and Concrete Association (GCCA) publishes a rating system, Low Carbon Concrete (or Cement) Ratings, which defines eight separate low-carbon grades (AA through G). These grades correspond to the depth of decarbonization and are based on a sliding scale related to the material’s compressive strength. When developing these criteria, Microsoft considered grades AA through D to be eligible for consideration in its EAC procurements.

Decarbonizing Steel

EACs can support transformative technologies for decarbonizing steel, like direct reduced iron (DRI) with electric arc furnaces (EAF) powered by green or blue hydrogen. This support can enable the steel sector to move beyond incremental improvements and focus on capital-intensive solutions that have the potential to reshape the industry. ​

ResponsibleSteel, an independent standards and certification organization with membership that is broadly representative of the steel industry, publishes decarbonization progress levels, with four separate grades (PL1–PL4). These grades correspond to the depth of decarbonization and are based on a sliding scale related to the fraction of recycled scrap use in the steel manufacturing process. When developing these criteria, Microsoft included levels PL2 through PL4 for consideration in its EAC procurements.

A Blueprint for EAC Market Development ​

Relae and Microsoft crafted Criteria for High-Quality Environmental Attribute Certificates in the Concrete and Steel Sectors as a public resource that can help shape the future of the commodity EAC market. ​

These criteria are designed to:

• Communicate intent: Communicate Microsoft's intent to support the decarbonization and scaling of physical commodity supply chains where they have been previously limited.‍
• Guide decision-making: Serve as a public guide for Microsoft's decision-making process, explaining the rationale behind pursuing or declining specific EAC pathways.‍
• Set high-integrity standards: Maintain high integrity for EACs at an early stage, ensuring they are used to enable future physical procurement by supporting market development.‍
• Stimulate partnership opportunities: Encourage potential partnerships and purchasing pipelines by clearly describing criteria for high-quality EACs.‍
• Publicly share detailed thinking: Build on previous work and publicly share detailed thinking, aiming to stimulate market development and guide procurement decisions.

Practical Recommendations for Implementing EACs

Whether you are a supplier, buyer, policymaker, or sustainability advocate, these new criteria offer actionable insights to help you navigate the complexities of EACs in the built environment. ​From setting significance thresholds to ensuring verifiability to avoiding double counting, the criteria provide clear guidance to maximize the impact of EAC-supported projects.

Beyond the technical aspects of EACs, these criteria aim to address their broader social and environmental implications. The criteria focus on the importance of community engagement, transparency, and equitable distribution of benefits in EAC project planning, ensuring that decarbonization efforts contribute to a more sustainable and just economy. ​The criteria are backed by extensive research, stakeholder consultations, and industry expertise.

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Frequently Asked Questions

What is an environmental attribute certificate (EAC), and how does it differ from buying low-carbon concrete or steel directly? 

An environmental attribute certificate represents the sustainability attributes of a commodity material such as concrete or steel, unbundled from the physical material and transacted separately from it. Direct procurement means a buyer purchases low-carbon concrete or steel and takes physical delivery of it for a specific project. An EAC purchase instead allows a buyer to fund and claim the environmental benefit of sustainably produced materials that are sold as typical commodities, and apply that benefit against conventional commodities in their own supply chain. EACs are priced to reflect the additional cost of producing the material sustainably, which sends a demand signal to producers. This makes EACs useful where market barriers—limited production volumes, geographic concentration of supply, or long-distance transportation—prevent buyers from sourcing low-carbon materials directly.

How do EACs for concrete and steel compare to renewable energy certificates (RECs) or sustainable aviation fuel (SAF) certificates?

Environmental attribute certificates for concrete and steel share a structure with renewable energy certificates and sustainable aviation fuel certificates: each decouples the environmental benefit of low-carbon production from the physical product, letting a buyer fund and claim that benefit without taking physical delivery. There are two key differences. First, conventional RECs represent electricity with a zero-emissions footprint, while every tonne of concrete or steel behind an EAC still carries embodied emissions — the certificate demonstrates reduction against a baseline, not a zero-emission product. Second, REC and SAF markets are established, while EACs for concrete and steel are nascent. Both differences are why rigorous criteria matter: quantifying the reduction credibly is harder, and the virtual structure carries risks of double counting, greenwashing, and fraud.

Will EACs let companies claim emissions reductions without actually reducing their built environment climate footprint?

An EAC purchase does not change the materials in a buyer's own buildings; it funds lower-emission production elsewhere in the market. Credible EAC frameworks therefore require that certificates complement rather than displace direct procurement, that they represent significant reductions against a defensible baseline, and that the funded project would not have proceeded without EAC revenue. The criteria published by Relae and Microsoft enable buyers to address these requirements directly in their procurement processes - addressing the climate and reputational risks of low-quality certificates. Used this way, EACs become an additional tool to address emissions from the built environment, supplementing design changes, direct procurement, and investment in new production pathways.

Who should use these criteria?

Criteria for High-Quality Environmental Attribute Certificates in the Concrete and Steel Sectors, co-developed by Relae and Microsoft, is a public resource intended for four audiences. Buyers pursuing scope 3 reductions can use it to evaluate whether a given certificate represents a credible, additional emissions reduction. Suppliers and project developers can use it to understand what purchasers of high-quality certificates look for before bringing a project to market. Policymakers and standards bodies can draw on it while shaping rules for an emerging market. Sustainability practitioners and advocates can use it as a reference point for assessing claims made about low-carbon concrete and steel. The criteria are designed to evolve as these sectors decarbonize, and users may adapt them to meet their own levels of capability and ambition. 

Key Takeaways

• Marine carbon dioxide removal (mCDR) uses ocean-based processes—primarily ocean alkalinity enhancement (OAE) and direct ocean removal (DOR)—to capture and durably store atmospheric carbon dioxide (CO₂), without the land and freshwater constraints of terrestrial methods. 
• The ocean already absorbs roughly 29% of human-caused CO₂ emissions each year, according to the 2025 Global Carbon Budget, and mCDR technologies aim to safely enhance that natural uptake without worsening ocean acidification.
• For corporate carbon buyers and project developers evaluating mCDR, the Criteria for High-Quality Marine Carbon Dioxide Removal, developed by Microsoft and Relae, outlines standards to guide responsible mCDR deployment at scale.

Reducing carbon dioxide (CO₂) emissions alone is no longer sufficient to limit global warming to 1.5°C. To effectively address climate change, scientific consensus highlights the need to remove carbon dioxide already present in the atmosphere. Marine carbon dioxide removal (mCDR), a form of ocean-based carbon removal, offers a scalable solution by leveraging the ocean’s natural ability to absorb and store carbon, without the land and resource limitations associated with terrestrial carbon removal methods.

This guide explores the fundamentals of marine carbon dioxide removal, including how it works, the technologies involved, and its role in global decarbonization.

What Is Marine Carbon Dioxide Removal?

Marine carbon dioxide removal uses ocean-based processes to capture and store carbon dioxide from the atmosphere. mCDR techniques fall into two primary categories: 

• Biotic CDR, which includes using photosynthetic fixation, microalgae cultivation, and terrestrial biomass sinking, to capture and store carbon. 
• Abiotic CDR, which influences CO₂ concentrations and carbonate chemistry in the seawater to absorb atmospheric CO₂ without increasing acidity. There are two main abiotic mCDR pathways: ocean alkalinity enhancement (OAE) and direct ocean removal (DOR). 

How Does Marine Carbon Dioxide Removal Work?

Marine carbon dioxide removal functions via the air-sea gas exchange—a process by which the atmosphere and surface seawater maintain equal CO₂ concentrations—meaning a shift in one leads to a corresponding change in the other. This exchange allows the ocean to absorb or release CO₂ back into the atmosphere depending on concentration levels and factors like pH. 

Once absorbed by the ocean, CO₂ exists in several forms, including dissolved CO₂, carbonates, and bicarbonates, influencing the ocean's pH levels. As atmospheric CO₂ levels rise, the oceans absorb roughly 12 GtCO₂ annually, approximately 29% of anthropogenic CO₂ emissions. This increase is disrupting the natural CO₂ balance and contributing to ocean acidification. 

mCDR methods like ocean alkalinity enhancement and direct ocean removal use safe and controlled processes to increase the amount of CO₂ absorbed from the atmosphere while mitigating acidification.

Understanding Ocean Alkalinity Enhancement (OAE)

Ocean alkalinity enhancement (OAE) captures and stores atmospheric CO₂ as dissolved carbonates in the ocean by increasing the alkalinity of seawater using one of two primary methods:

• Adding alkaline minerals such as olivine or basalt to seawater.
• Using electrochemical methods to add alkaline compounds to seawater.

Both of these methods increase the alkalinity of seawater and, thereby, its capacity to absorb atmospheric CO₂ without acidification. 

OAE methods can also differ by location and the manner in which seawater interacts with alkaline substances and atmospheric CO₂. This interaction may occur in the open ocean or within a controlled mCDR project facility. Each approach involves trade-offs: open ocean methods tend to be less energy-intensive but present greater uncertainties and challenges in measurement, reporting, and verification (MRV).

Understanding Direct Ocean Removal (DOR)

Direct ocean removal (DOR), also called direct ocean capture (DOC), removes CO₂ dissolved in seawater using the controlled acidification of seawater in a closed system using one of two primary methods: 

• Electrochemical methods such as electrolysis of seawater or electrodialysis
• The addition of minerals like olivine and basalt. 

The acidified and CO₂ depleted seawater is neutralized to native pH and allowed to equalize with and remove atmospheric CO₂. Once removed, CO₂ can be safely stored using geologic storage with a durability of >1,000 years.

Key Benefits of Marine Carbon Dioxide Removal

Understanding how mCDR works highlights its potential to address climate challenges. Here are the key benefits that make it a critical tool in global decarbonization strategies.

• Scalable carbon removal without land constraints: Marine carbon dioxide removal does not require large land areas or significant freshwater resources, making it highly scalable.
• Harnessing the ocean’s natural carbon sink: The ocean absorbs about 25% of human-generated CO₂ annually. mCDR enhances this natural process, increasing carbon storage without accelerating ocean acidification.
• Diverse technological pathways for flexibility: Technologies like ocean alkalinity enhancement and direct ocean removal offer flexible solutions tailored to different environments and project needs.
• Global reach with a large surface area: Covering over two-thirds of the Earth’s surface, the ocean provides an expansive platform for mCDR technologies globally. 
• Potential to mitigate ocean acidification: Some mCDR methods, such as ocean alkalinity enhancement, not only remove CO₂ but also help restore ocean pH levels, supporting marine ecosystem health.

Challenges of Marine Carbon Dioxide Removal

While mCDR holds significant promise, it also presents challenges that must be addressed for responsible deployment.

• Technical scalability and efficiency: Scaling mCDR technologies to achieve meaningful carbon removal while maintaining energy efficiency remains a significant hurdle.
• Potential environmental impacts: Altering ocean chemistry may pose risks to marine ecosystems, with long-term effects still not fully understood.
• Measurement, reporting, and verification (MRV) complexity: Accurately measuring CO₂ removal and ensuring its durability requires advanced monitoring systems, which are still evolving.
• Regulatory and governance gaps: Clear global policies are needed to oversee mCDR deployment, manage environmental risks, and ensure accountability.
• Public perception and ethical considerations: Concerns around geoengineering and potential unintended consequences may impact public acceptance and policy support.

Deploying Marine Carbon Dioxide Removal 

As mCDR technologies evolve, effective deployment will rely on adaptive management practices to address technical, environmental, and regulatory challenges. This includes robust MRV systems for accurate CO₂ removal tracking and continuous ecosystem monitoring to mitigate potential risks to marine life. 

Collaborative efforts between scientists, policymakers, and project developers are key to establishing clear regulatory frameworks, optimizing technologies for efficiency and scalability, and building public trust. These practices ensure mCDR can be scaled responsibly while safeguarding ocean health. 

The Future of Marine Carbon Dioxide Removal 

As technologies like ocean alkalinity enhancement and direct ocean removal advance, their potential to deliver large-scale, durable carbon removal is becoming increasingly evident. Realizing this potential requires more than technological innovation—it depends on rigorous environmental monitoring, transparent reporting, and strong collaboration among project developers, carbon buyers, and policymakers.

Establishing clear, consistent standards for high-quality mCDR is essential to ensure both climate effectiveness and environmental safety. To support this, Microsoft and Relae have partnered to develop the Criteria for High-Quality Carbon Dioxide Removal. 

Frequently Asked Questions

What is marine carbon dioxide removal (mCDR)?

mCDR uses ocean-based processes, primarily ocean alkalinity enhancement and direct ocean removal, to capture and durably store atmospheric CO₂. It works by increasing the ocean's natural capacity to absorb CO₂ without increasing acidification.

How does marine carbon dioxide removal compare to land-based methods like direct air capture or reforestation?

Unlike land-based approaches, mCDR doesn't require large land areas or freshwater, and the ocean's size gives it significant scaling potential. It's earlier-stage than more established pathways, though, with measurement and environmental monitoring standards still maturing.

Is marine carbon dioxide removal proven and scalable today, or still emerging?

mCDR technologies have shown promising results in lab testing and early deployments, but confirming safety and effectiveness at a large real-world scale requires more monitoring data. It's best described as an emerging pathway with strong near-term momentum, not yet a mature, at-scale solution.

What should a company look for when evaluating a marine carbon dioxide removal project or credits?

Buyers should look for rigorous carbon MRV paired with equally rigorous monitoring of ocean ecosystem health (eMRV), transparent reporting, and adherence to established frameworks like the Criteria for High-Quality Carbon Dioxide Removal.