Pragmatic balancing of engineered and nature-based solutions for effective carbon dioxide removal (CDR)

Net-zero, eventually

The challenge of anthropogenic climate change places unprecedented stress on both planetary and human systems through myriad exposure pathways. A paradigm shift toward comprehensive mitigation and adaptation strategies is imperative to navigate this crisis.

Key to this transition is a three-pronged approach:

1. Minimizing reliance on fossil carbon by curtailing carbon-intensive activities.
2. Transitioning to renewable energy sources replaces fossil carbon energy with sustainable alternatives.
3. Enhancing process efficiency and leveraging appropriate institutional incentives to diminish greenhouse gas (GHG) emissions.

Yet, even with these strategies, we are confronted with the inevitability of residual emissions. The quest to achieve net zero thus necessitates high-quality carbon removal interventions, exemplified by initiatives like Puro.earth and Verra, which spearhead the capture of these lingering emissions.

The strategic removal of atmospheric carbon is central to any realistic pathway toward net-zero ambitions. This is particularly critical for counterbalancing residual emissions from sectors like cement and steel production, where emissions are deeply entrenched, and current technologies need to achieve complete decarbonization with significant advancements in carbon capture and storage (CCS) capabilities.

According to a report by the UK National Infrastructure Commission, a collective effort to remove between 40 and 100 Megatons of CO?-equivalent (MtCO?e) annually is essential. This volume equates to 1 to 2.5 years of global carbon emissions from fossil fuels, signaling a need for Moonshot-level innovation in solution scaling.

Carbon dioxide removal (CDR) solutions landscape

The debate around the most cost-efficient, scalable, and mature carbon removal solutions is ongoing, with two primary categories emerging:

• Nature-based solutions (NbS): These strategies focus on the conservation, restoration, and improved management of natural ecosystems that sequester carbon, ranging from mangrove conservation and peatland preservation to regenerative agriculture. In 2023, natural sinks absorbed an estimated 23.8 Gigatons of CO?-equivalent (GtCO?e), highlighting their significant role in climate mitigation (Global Carbon Atlas, 2023).
• Engineered-based solutions: Technological approaches to carbon removal, such as bioenergy with carbon capture and storage (BECCS), direct air capture with carbon storage (DACCS), and ocean alkalinization, offer man-made solutions to the carbon challenge. Despite their nascent stage, these technologies absorbed 10,000 tons of CO?e in 2023, both a mere and encouraging figure regarding their development cycle (R.Jackson, 2023).

Engineered CDR

As we advance, the focus must remain on relentless R&D for promising engineered-based carbon removal solutions and the strategic deployment of NbS by creating conducive incentives for project development.

A National Geographic article from October 2023 illustrates the disparity in removal potential, complexity, and cost among these solutions. Direct air capture (DAC), despite being the most complex and expensive, is identified as an up-and-coming solution, with companies like Climeworks making significant strides.

A recent report by the U.S. Government Accountability Office assigns DAC a technology readiness level (TRL) of 7 out of 9, indicating its maturity and highlighting cost as a significant hurdle. The economic challenge is evident with DAC costing approximately 700 USD per ton of CO? compared to reforestation's 25 USD per ton.

The graph made by a recent MSCI Carbon Markets illustrates a striking price premium for engineered CDR, with Direct Air Capture (DAC) leading to costs significantly higher than other approaches. This premium reflects the advanced technology and energy requirements of engineered CDR processes. While also an engineered solution, biochar is presented as a more cost-effective option yet still priced higher than most nature-based and mixed solutions. This disparity underscores the need for continued innovation and investment to reduce costs and improve the scalability of engineered CDR methods, ensuring their economic viability as part of a comprehensive strategy for carbon reduction.

Despite the cost barriers associated with engineered CDR solutions, their appeal lies in their higher assurance regarding additionality and permanence. However, for DAC to be deemed economically viable, efficiency enhancements are necessary due to the energy-intensive nature of capturing CO? from the atmosphere's low partial pressure. A groundbreaking study published on March 1, 2024, by Bjarne Steffen, Katrin S., and Tobias Schmidt from ETH Zürich introduces an innovative approach to forecasting the costs of emerging technologies. Utilizing a techno-economic model, their research evaluates three DACCS (Direct Air Carbon Capture and Storage) technologies, offering a probabilistic forecast for future pricing. Their findings indicate that, across all scenarios analyzed, neither liquid nor solid DACCS technologies are expected to achieve the cost target of $100 per ton of CO?, raising concerns about the influence of such cost expectations on market dynamics.

Nonetheless, the principle of economies of scale suggests promising avenues for cost reduction as technology deployment progresses. The study forecasts that at a cumulative capacity of 1 gigaton of CO? per year—a highly ambitious target—DACCS costs could be as low as $341 per ton of CO? ($226–$544 with 90% confidence) for the most cost-effective technology among those studied, namely liquid solvent DACCS. This projection implies that cost reductions may not be as substantial as anticipated. However, DACCS technologies could still become financially viable options within hard-to-abate sectors and align with the marginal cost of abatement (MCA) projections by 2050. The accompanying figure illustrates this concept, indicating that despite the anticipated high cost per ton, DACCS might achieve cost-competitiveness, especially when considering projected European Union's Emission Trading Scheme (ETS) carbon prices for 2050 and the associated costs of abating emissions in the aviation sector.

A case for biochar

Biochar emerges as a compelling candidate within the circular economy framework, standing out as one of the most cost-effective engineered solutions for DAC. The term "biochar" itself is a fusion of "biomass" and "charcoal," reflecting its origins and production process, which unfolds through three primary stages:

1. Sourcing biomass: The biomass used for biochar production can vary widely, including agricultural residues (e.g., straw, husks), forestry waste (e.g., wood chips, sawdust), and organic waste from urban sources. This versatility underscores biochar's sustainability and its role in waste reduction and resource utilization.
2. Transport and processing: Biomass is transported to the production facility for preprocessing (e.g., drying, shredding, pelletizing).
3. Production: The biomass undergoes pyrolysis —a thermochemical decomposition process occurring at high temperatures (300°C to 700°C) in an oxygen-deprived environment. This process transforms biomass into biochar, bio-oil, and syngas. The specific conditions (temperature, heating rate, and residence time) significantly influence the yield and properties of the biochar produced.

The flow chart below from a life-cycle analysis (LCA) of biochar production from Carbofex provides the system boundary from input - biomass - to output - biochar and co-products.

This process generates biochar and co-products such as liquids, i.e., bio-oil, and gases, i.e., syngas. Biochar can be used in soil amendment. These co-products can be harnessed for energy, further pyrolysis (recovery), or as inputs in other industrial processes, thereby creating a closed-loop system that minimizes waste and maximizes resource efficiency.

Biochar's application to soil not only sequesters carbon for centuries, mitigating climate change, but also enhances soil health, water retention, and agricultural productivity. This dual functionality—carbon sequestration and soil amendment—positions biochar as a uniquely beneficial product within the spectrum of engineered DAC solutions, offering a sustainable, cost-effective ($????140/tCO?) method for capturing carbon while contributing to agricultural and environmental sustainability.

What makes biochar a high-quality engineered solution in the carbon market?

In the context of the carbon market, biochar stands out as an exemplary engineered solution, distinguished by its robust alignment with critical carbon credit criteria such as additionality, quantification, permanence, and the realization of co-benefits, albeit with certain delivery risks that warrant attention.

• Additionality: Biochar production uniquely contributes to carbon sequestration by transforming biomass that would otherwise decompose and emit CO? and methane into a stable form of carbon. This process embodies the principle of additionality, offering new and measurable reductions in atmospheric carbon levels beyond what would occur in its absence.
• Quantification: The carbon sequestration benefits of biochar are readily quantifiable, facilitating the generation of verifiable carbon credits. This quantification is supported by rigorous methodologies that account for the carbon stored in biochar, ensuring that credits represent genuine emissions reductions.
• Permanence: One of biochar's most compelling attributes is the longevity of its carbon sequestration, with the potential to lock away carbon for centuries. This high degree of permanence addresses a critical aspect of carbon credit quality, offering long-term climate benefits.
• Co-benefits: Beyond its direct climate mitigation impact, biochar production delivers significant co-benefits, including enhanced soil health, improved water retention, reduced need for chemical fertilizers, and lower emissions of other greenhouse gases. These environmental and agricultural co-benefits enrich the value of biochar-derived carbon credits.

Finally, the economic viability of biochar, supported by its scalability, makes it an accessible solution for many stakeholders. The accompanying graph shows the biochar market, demonstrating its diverse revenue potential across various applications and market maturities. It shows that while biochar can fetch a high price as animal feed, with values exceeding $1,000 per ton of CO?e, it also has considerable potential in other areas such as fertilizers, soil amendments, and as a construction filler, with prices ranging from $100 to $500 per ton of CO?e. Interestingly, the sale of carbon credits for biochar, which can serve as a significant incentive for producers, sits at a moderate price point between $100 to $200 per ton of CO?e, competitive with the costs of other carbon reduction methods. This pricing structure highlights biochar's versatility and the importance of market maturity in realizing its full economic potential. It suggests that as the biochar market matures, there may be substantial room for growth in revenue streams, making it an increasingly attractive option in the carbon market landscape.

Elevating nature-based CDR

As climate change intensifies, expanding nature-based solutions (NbS) for carbon dioxide removal becomes increasingly paramount. NbS presents a cost-effective, readily available strategy to mitigate residual emissions by leveraging the innate capabilities of natural ecosystems.

A pivotal study from 2021 highlights the potential of land-based carbon removal, estimating an affordable (costing up to $100 per ton of CO?) capacity of 8-13.8 GtCO? eq annually, with enhanced land sequestration emerging as the frontrunner. The beauty of NbS lies in their utilization of the natural carbon-sequestering power of forests, wetlands, oceans, and soils. These mechanisms contribute to the reduction of atmospheric CO? levels and bring forth extensive co-benefits, including biodiversity conservation, water quality improvement, and the bolstering of local economies.

Scaling up nature-based CDR

Achieving the necessary expansion and impact of NbS requires a concerted effort in research and development, policy reinforcement, and the provision of financial incentives. Enhancing the appeal and feasibility of NbS projects hinges on the development of robust carbon markets, both compliance and voluntary, offering a financial backbone to support these initiatives.

Incorporating NbS into comprehensive climate strategies ensures an integrated approach to environmental stewardship, encompassing carbon capture, ecosystem resilience, and restoration. By prioritizing nature-based approaches, we can forge a more robust and predictable path towards climate mitigation that capitalizes on the natural world's offerings.

Guiding Principles for NbS Implementation

Furthermore, integrating NbS into broader climate action strategies ensures a holistic approach to sustainability, addressing carbon sequestration, ecosystem preservation, and restoration. By prioritizing the deployment of NbS, we can achieve a more predictable climate mitigation strategy that leverages the best of what nature offers.

Additionally, a recent article published by Nathalie Seddon et al. (2021) underscores four critical considerations to ensure that NbS deliver net positive outcomes as they gain prevalence:

1. Fossil fuel phase-out complement: NbS should complement, not replace, the urgent elimination of fossil fuel usage.
2. Ecosystem diversity: Effective NbS encompass a diverse array of ecosystems, extending beyond forests to include marine and other terrestrial environments.
3. Community engagement: The successful implementation of NbS mandates active involvement and consent from Indigenous Peoples and local communities, safeguarding their cultural and ecological rights.
4. Biodiversity and benefits: NbS must be purposefully designed to yield quantifiable benefits for biodiversity alongside carbon sequestration.

Market-driven viability of NbS

The market attractiveness of NbS for carbon dioxide removal is significantly enhanced through the support of carbon markets. These platforms offer financial incentives to project developers while ensuring credibility and transparency for purchasers. The Integrity Council for the Voluntary Carbon Market (ICVCM) is pivotal in advancing NbS within these frameworks by establishing stringent standards and verification processes. Their efforts aim to connect NbS projects with potential investors, thereby catalyzing the flow of capital towards impactful and sustainable climate actions. The ICVCM’s 10 Core Carbon Principles stand as a testament to pursuing higher-quality carbon credits.

Concluding thoughts

The challenge of achieving net-zero emissions hinges on a dual approach: curtailing emissions at their source and enhancing atmospheric carbon dioxide removal. This strategy delineates two paths: leveraging nature-based solutions (NbS) like peatlands and wetlands for their carbon sequestration capabilities and deploying engineered carbon dioxide removal (CDR) technologies such as biochar. While NbS offers a cost-effective and immediately actionable route with added biodiversity and community benefits, its scalability depends on significant investments and supportive policies. Conversely, engineered CDR technologies, despite facing challenges related to cost and developmental stages, represent a frontier of innovation with the potential to complement NbS in carbon removal efforts.

A balanced integration of nature-based and engineered solutions is essential for net-zero strategies. The interplay between leveraging the earth's natural carbon sinks and developing technological solutions is what we ought to do and try. As we progress, ensuring the quantification, permanence, and maximization of co-benefits across these solutions will be crucial to the success of our global climate mitigation efforts.