What are the biggest scientific uncertainties around durable carbon removal?

How long does carbon actually need to stay stored to be 'durable'?

The definition of 'durable' is itself a major uncertainty. A 2024 study using a reduced-complexity climate model found that if removed CO₂ is stored for only 100 years—a common benchmark—and residual emissions continue at 6 billion tons per year, the planet would see an additional 0.8°C of warming by 2500 compared to permanent storage [6]. The same study concluded that storage periods under 1,000 years are insufficient to neutralize remaining fossil CO₂ emissions under net-zero frameworks [6]. This means that many land-based methods, like soil carbon sequestration or afforestation, which store carbon for decades to centuries, may not qualify as truly durable unless they can demonstrably exceed that millennium threshold.

The challenge is that different methods have vastly different storage timescales. Soil organic carbon has a turnover time of decades to centuries, while inorganic carbon (e.g., bicarbonates from enhanced weathering) can persist for thousands of years [1]. Wood burial, as proposed in the 'Wood Vault' concept, aims for semi-permanent storage in anaerobic conditions, but the authors acknowledge that durability depends heavily on the specific burial environment and method [2]. This variability makes it hard to compare or certify different approaches.

Can we actually measure and verify durable carbon storage at scale?

A core uncertainty is the lack of reliable, standardized measurement protocols. For enhanced weathering—spreading crushed rock on croplands to accelerate CO₂ absorption—a 2024 review noted that diverging methods for determining weathering rates make it nearly impossible to compare results across studies [1]. In a controlled pot experiment, the measured carbon capture was just 0.01 tons of CO₂ per hectare per year, at the low end of previous estimates, and the authors called for a consistent protocol to verify carbon dioxide removal (CDR) potential [1].

For marine carbon dioxide removal (mCDR), the problem is even starker. A 2025 perspective paper argued that because the ocean carbon pool is so vast and variable, observations alone cannot resolve how much additional carbon a deployment actually sequesters versus a no-intervention scenario [5]. Ocean biogeochemical models are expected to fill this gap, but current models lack the process representations needed to simulate perturbations at deployment scales [5]. This means we cannot currently verify the durability of ocean-based methods with confidence.

Across all methods, a 2026 life-cycle assessment that harmonized data from multiple sources found that uncertainty bands are wide enough to affect technology ranking: biochar costs $140 ± 20 per ton of CO₂, while direct air capture costs $640 ± 120 [4]. The authors stress that comparing technologies with quantified uncertainty is essential for decision-making, but current data often lack this rigor.

What are the hidden trade-offs and side effects of different methods?

Even promising methods come with uncertainties about side effects that could undermine their net benefit. In enhanced weathering, a 2024 experiment found that while rock dust increased soil inorganic carbon by 46% (a tiny absolute increase of 0.0002%), it also raised soil pH, which reduced availability of micronutrients like manganese and zinc to plants [1]. This forced plant roots to exude organic acids to solubilize those nutrients, which in turn destabilized soil organic carbon aggregates, partially offsetting the carbon gain [1]. The authors noted a lack of direct evidence for this plant response, adding uncertainty to the plant-rock-carbon nexus [1].

Land-based methods also face hard trade-offs. A 2025 portfolio optimization model showed that bioenergy with carbon capture and storage (BECCS) is severely constrained by available land: its median deployment falls from 1.8 to 0.3 billion tons of CO₂ per year in land-constrained scenarios [3]. Afforestation and reforestation appear robust, but a 2024 framework paper warned that these 'nature-based solutions' can have adverse effects on biodiversity and ecosystem services if not implemented with ecosystem integrity and human rights in mind [7]. The same paper emphasized that climate mitigation efforts must not compromise biodiversity or human well-being, adding social and ecological dimensions to the uncertainty.

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This answer is built on 7 peer-reviewed studies — published from 2022 to 2026, 6 from 2024 or later, 4 in Q1 journals, collectively cited 65 times — selected as the most relevant from 7 studies that passed quality screening, drawn from 50 papers retrieved from a database of over 500 million.

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