Carbon Removal Permanence: Ensuring Long-Term Climate Benefits

Quick verdict

Carbon removal permanence is a measure of how long removed CO2 stays out of the atmosphere. It is a critical factor in evaluating carbon removal solutions because temporary storage may not provide lasting climate benefits. High-permanence methods like geological storage and mineralization are essential for long-term climate goals, but they are often more expensive and less scalable in the short term. A portfolio of approaches with varying permanence may be necessary, but robust monitoring and accounting are required to ensure climate integrity.

Problem addressed

The accumulation of CO2 in the atmosphere is the primary driver of climate change. To meet the Paris Agreement goals, not only must emissions be drastically reduced, but large-scale carbon dioxide removal (CDR) is also needed. However, if removed carbon is not stored durably, it can re-enter the atmosphere, negating the climate benefit. The problem of impermanence undermines the effectiveness of CDR and creates risks for carbon markets and climate policy. Ensuring that removed carbon stays out of the atmosphere for centuries to millennia is essential to stabilize the climate.

How the solution works

Permanence in carbon removal is achieved through different storage mechanisms. Biological storage (e.g., forests, soils) sequesters carbon in biomass and organic matter, but it is vulnerable to reversal from disturbances like fire, pests, or land-use change. Geological storage involves injecting CO2 into deep rock formations (e.g., saline aquifers, depleted oil and gas reservoirs) where it is trapped by impermeable caprock and mineralizes over time, potentially lasting thousands to millions of years. Mineralization accelerates natural weathering processes to convert CO2 into stable carbonate minerals, offering effectively permanent storage. Ocean-based methods aim to store carbon in the deep ocean or as dissolved bicarbonate, with residence times of centuries to millennia. Ensuring permanence also requires robust monitoring, reporting, and verification (MRV) systems to detect and quantify any leakage or reversal, and to assign liability for potential releases.

Evidence strength

The scientific understanding of permanence varies by method. Geological storage is supported by decades of experience in enhanced oil recovery and natural gas storage, as well as pilot projects like Sleipner in Norway, where injected CO2 has been monitored since 1996 without significant leakage. Mineralization is well-understood in laboratory and pilot settings, but large-scale deployment data are limited. Biological storage permanence is less certain due to the complexity of ecosystems and the increasing risks of climate-related disturbances (e.g., wildfires, pest outbreaks). Ocean-based methods are still largely in the research phase, with permanence estimates based on ocean chemistry models. Overall, the evidence base for high-permanence methods is moderate to strong, but long-term field data are scarce for most approaches.

Potential scale

The scale of permanent carbon removal is constrained by the availability of suitable storage sites and the energy and resource requirements of the removal process. Geological storage capacity is estimated to be vast (thousands of gigatons) but is unevenly distributed and requires significant infrastructure. Mineralization could theoretically scale to billions of tons per year if mining and processing of reactive minerals (e.g., olivine) can be expanded sustainably. Biological methods have large theoretical potential but are limited by land availability and competition with food production, and their permanence is lower. The overall scale of permanent CDR needed to meet climate targets is on the order of 10 GtCO2 per year by mid-century, which will require a portfolio of approaches.

Cost considerations

There is generally a trade-off between permanence and cost. Low-permanence methods like afforestation and soil carbon sequestration are relatively cheap ($10–$50 per ton of CO2) but carry high reversal risk. High-permanence methods like direct air capture with geological storage (DACCS) are currently expensive ($100–$600 per ton) but offer durable storage. Mineralization and bioenergy with carbon capture and storage (BECCS) fall in between. The cost of monitoring and verification adds to the total. Carbon markets are beginning to differentiate prices based on permanence, with higher prices for more durable credits.

Implementation time

Deployment timelines vary. Afforestation can begin immediately but takes decades to reach full carbon storage potential and is subject to reversal. Geological storage requires site characterization, permitting, and infrastructure build-out, which can take 5–10 years. DACCS and mineralization facilities can be built in a few years but scaling to climate-relevant levels will take decades. Monitoring for permanence is an ongoing, long-term commitment.

Environmental benefits

The primary environmental benefit of permanent carbon removal is the reduction of atmospheric CO2 concentrations, which mitigates climate change and its associated impacts (e.g., sea-level rise, extreme weather). Some methods offer co-benefits: afforestation can enhance biodiversity and water regulation; mineralization can improve soil quality; DACCS has minimal direct environmental impact if powered by renewables. However, large-scale deployment of any method may have land, water, and energy footprints that need careful management.

Social and economic co-benefits

Permanent carbon removal industries could create jobs in engineering, construction, monitoring, and research. They may also stimulate innovation and provide revenue streams for communities hosting storage sites. In some cases, co-benefits like improved air quality from reduced fossil fuel use (if CDR is coupled with decarbonization) can yield health benefits. However, these benefits are not automatic and depend on policy design and community engagement.

Risks and unintended consequences

The main risk is leakage or reversal of stored carbon, which could be triggered by natural events (earthquakes, wildfires) or human activities (poor well integrity, land-use change). There is also a moral hazard: the promise of future permanent removal might delay emissions cuts. Large-scale deployment could compete for land, water, and energy, potentially driving up food prices or causing ecological harm. Public opposition to geological storage sites (e.g., perceived risk of induced seismicity) could impede deployment. Additionally, the long-term liability for stored CO2 is unresolved.

Where it works best

High-permanence methods are most suitable where there is a need for durable, verifiable carbon removal to offset persistent emissions (e.g., from aviation or industry) or to achieve net-negative emissions. Geological storage works best in regions with suitable sedimentary basins (e.g., North Sea, Gulf of Mexico, Middle East). Mineralization is well-suited to areas with abundant reactive rocks (e.g., Oman, Norway). DACCS can be located anywhere with access to low-carbon energy and storage. Low-permanence methods may be appropriate for interim climate mitigation if they buy time for permanent solutions to scale.

Where it may not work

In regions without suitable geology or where public acceptance is low, geological storage may not be feasible. Temporary storage methods are not a substitute for permanent removal when offsetting fossil fuel emissions, as the carbon debt is effectively permanent. In areas with high risk of natural disturbances (e.g., fire-prone forests), biological storage may be unreliable. Permanence also requires robust governance and monitoring, which may be lacking in some jurisdictions.

Comparison with alternatives

The main alternative to permanent carbon removal is temporary storage, which is cheaper but less effective in the long run. Another alternative is to focus solely on emission reductions; however, most climate scenarios require some CDR. Within CDR, methods can be compared on a matrix of permanence, cost, scalability, and co-benefits. No single method scores highest on all dimensions, so a portfolio is often recommended. Some propose that temporary storage can be valuable if it is used to offset short-lived climate forcers or if it is paired with a commitment to replace it with permanent storage later, but this introduces complexity and risk.

Case studies

The Sleipner project in Norway has been injecting CO2 into a deep saline aquifer since 1996, with extensive monitoring showing no leakage, demonstrating the feasibility of long-term geological storage. Climeworks’ Orca plant in Iceland captures CO2 from air and injects it into basalt formations where it mineralizes within a few years, providing a real-world example of permanent removal via DACCS and mineralization. The California Air Resources Board’s forest offset protocol includes a buffer pool to insure against reversal, but recent wildfires have raised concerns about the adequacy of such mechanisms. The CarbFix project in Iceland has demonstrated rapid mineralization of CO2 in basalt, turning it into stone within two years, confirming the permanence of this approach.

Final assessment

Carbon removal permanence is a non-negotiable requirement for CDR to meaningfully contribute to climate stabilization. While no method is perfectly permanent, geological storage and mineralization offer the highest confidence in multi-century to millennial storage. The current portfolio of CDR methods is skewed toward lower-permanence, cheaper options, but scaling high-permanence methods is essential. Policy and market mechanisms must incentivize durability and ensure robust monitoring. A responsible approach combines aggressive emission reductions with a rapid scale-up of permanent CDR, while using temporary storage only as a bridge with clear limits and accountability.