· 6 min read
Carbon dioxide removal (CDR) has been much in the news of late, with the U.S. Department of Energy’s announcement in August 2023 of the first $1.2B of $3.5B eventual funding for Direct Air Capture (DAC) Hubs. Alongside a strong prioritization of deep reductions of global greenhouse gas emissions, the 2023 Intergovernmental Panel on Climate Change Synthesis Report has stated that gigatonne-scale carbon dioxide removal (CDR) is an “unavoidable” component of the climate mitigation necessary to reach net zero by 2050 and have any chance to limit warming to 1.5 or even 2°C. The website cdr.fyi reports that (as of October 2023) total delivery of CDR to date is approximately 114,000 tonnes – an important start, but a far cry from what is necessary.
Scaling high-quality CDR to the multiple gigatonnes that will be required annually by mid-century will be a massive task. Success will require a portfolio approach that encompasses not only DAC but also land-based soil and forest carbon sinks; biomass-based carbon removal and storage (BiCRS); marine carbon dioxide removal (mCDR); mineralization-based approaches – as well as emergent and potentially as yet undiscovered methods. It is critical that public policy, R&D funding, and private-market buyers consider and support this full range of approaches, particularly those with the potential to scale quickly, in the immediate term, and at low cost.
Introducing wood harvesting and storage
Wood harvesting and storage (WHS) is a novel BiCRS method that meets these scaling and cost criteria. WHS entails collecting residual woody biomass (e.g. from ecological thinning and other forest management activities) and storing it in a Wood Vault, an underground burial chamber specifically engineered to preserve the biomass for centuries to millennia. WHS offers a number of important advantages, including low-cost potential, minimal infrastructure requirements, and high carbon storage efficiency. Importantly, and unlike many other CDR methods, there are few physical impediments to scaling WHS. Feedstock is plentiful in the near term – 2-3Gt/y wood residuals globally (Zeng and Hausmann, 2022), and up to 2Gt standing stock of accessible, unmerchantable forest residues slated for fire-thinning over the coming decade in the western zone of the U.S. National Forest System alone (Crotty et al., 2023). WHS requires no new technology. Wood Vaults do not require centralized plants or processing facilities but rather can be sited in a distributed fashion to minimize the transport distance of feedstock.
As we look to advance WHS, we must acknowledge that the method is untested at scale as a long-duration CDR, and it is critical that we are led by science. For this reason, the authors have published Implementation Guidance for WHS Version 1.0, an open-source paper that we hope can provide science-based guidance for early WHS project deployment, as well as a foundation for an eventual global public standard for WHS that can serve as a basis for the operating protocols and methodologies that provide science-based verification for delivery of larger projects as the method scales.
WHS and the question of durability
At the core of WHS is durability: namely, how long can the carbon be stored in a Wood Vault. In the natural environment, dead wood will decay from within a few years to a few decades. To achieve our climate goals, carbon needs to be stored for hundreds to thousands of years or longer. The goal of Wood Vault design is to maximize wood durability by ensuring one or more of these conditions are met: (1) anoxic, (2) cold, and (3) dry conditions.
The current focus of WHS is on underground vaults that create anoxic conditions because anoxia appears to be an effective, widely applicable, and affordable means of preserving wood at a large scale. The key processes within anoxic wood vaults are:
- Anoxia is maintained by using clay barriers and low-permeability soil, while residual O2 is rapidly consumed by aerobic heterotrophic microbes. Anoxia within Wood Vaults precludes fungi, insects, and microbes that normally decompose wood.
- Within anoxic vaults, anaerobic bacteria are expected to dominate wood decay, but bacterial wood decay is slow and incomplete since lignin is resistant to anaerobic bacteria and physically protects holocellulose. As a result, long-term preservation is favored.
- Anoxic decay processes may promote methane generation. However, the potential for CH4 production is expected to be low. Since wood is nutrient-poor and lignin hard to digest, the activity of methanogens is typically low, as demonstrated in anaerobic bioreactor and landfill studies. Moreover, CH4 produced in the Wood Vault will be partially or mostly oxidized by methanotrophic bacteria as it diffuses through the soil cover to the surface, thereby minimizing or eliminating entirely the potential for CH4 flux from vaults.
Research, regulatory, and funding priorities for scaling WHS
While scientific theory and experimental evidence strongly support the feasibility of WHS, further research is needed on durability, potential methane emissions, and sustainable biomass sourcing. High-quality monitoring, reporting, and verification (MRV) of field projects will be essential to ensure the method’s effectiveness with minimal negative impact.
The first WHS demonstration projects will break ground in the coming months, with a strong focus on MRV and maximizing research and scientific learning. It will be important to assess WHS performance under a variety of environmental conditions and with a range of feedstock and storage types. These early projects will be important testbeds for the efficacy of the pathway.
With these projects coming online, important considerations remain for scalability. These include:
- Research on control of decay mechanisms: The durability of WHS depends on the level to which the activity of decomposing biota can be eliminated or reduced. Continued research on the mechanisms of wood decay and its control (via storage site design, bioengineering, wood treatments, etc.) is needed.
- Development of a regulatory pathway for WHS: In the United States, WHS facilities do not neatly fit into existing regulatory pathways. Regulation of WHS facilities should establish reasonable, performance-based criteria for water resources protection considering the biogeochemical character of natural wood.
- Funding for demonstration projects: In parallel with regulatory and research needs, policymakers can deploy the first generation of Wood Vaults in the United States using grant, research, and demonstration funding.
For the world to meet its climate goals, a diversified portfolio of high-quality CDR solutions will be required, and we must start deploying them now to determine which approaches should be scaled over the coming decades. WHS is a highly promising BiCRS method that we believe has an important climate role to play this century, and beyond.
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