Permanence Is Foundational to Carbon Removal 

Permanence is a cornerstone of high-quality carbon removal. It reflects the amount of time that carbon is expected to remain stored with low risk of reversal—and for carbon removal to match the climate impact of emissions, that timescale must extend beyond several centuries. 

For biochar, a scalable and proven carbon removal pathway, the science behind permanence has been clear: biochar is a worthwhile climate investment. Recent scientific developments have presented advances to established quantification approaches and increased confidence that biochar carbon storage can be guaranteed for several centuries. In parallel, emerging approaches to permanence have been put forward, discussing whether millennia persistence could potentially be guaranteed. 

At Puro.earth, we believe that applying science where it’s strong—and acknowledging where more data is needed—is fundamental to building trust. That’s why we decided to increase biochar durability to several centuries (CORC200+) in our updated methodology (Edition 2025), but not yet to millennia durability. Furthermore, we’re also convening a cross-standard workshop to help align on the future of biochar permanence quantification, and to collectively reflect on where consensus is forming, and where new methods may still be maturing. In this piece, we provide more insight on how biochar science has evolved, and what steps lie ahead of us. 

Biochar Is Scaling—And So Is the Science 

Since 2019, Puro.earth has certified over 70 biochar-based carbon removal projects. Biochar combines durable carbon storage with regional applicability and co-benefits, all supported by a growing body of science. As the market scales, so does the depth and diversity of biochar research. To understand the current landscape, it’s useful to review how permanence science has evolved over the last 25 years and how it informs certification today: 

Early Concept and Emergence (1990s–2000s) 
Biochar emerged in the late 1990s with investigations of dark earths in certain Amazonian soils. Soon, the idea is formulated that it can be a method for soil carbon storage over long periods, contributing to climate change mitigation and delivering agronomic benefits. Early research focused on agronomic benefits, with permanence being understood through analogies to charcoal and fire-derived pyrogenic matter. 

In the 1990s–2000s, the carbon removal market was still virtually non-existent, and biochar remained outside mainstream climate policy. 

Rising scientific interest and first persistence models (2000s–early 2010s) 
During this period, the number of scientific publications on biochar rapidly increased, primarily around agronomic effects, production techniques and property characterization, as well as persistence via multi-year decomposition experiments. The early 2010s saw the first publications correlating persistence to biochar properties and environmental factors. In particular, the molar H/C ratio and O/C ratio, which have long-been used as indicators of aromaticity in carbonized biomass, are suggested as indicators of biochar stability in soil. Numerous other metrics and characterization techniques are also studied (e.g. carbonization temperature, recalcitrance index, and various spectroscopy methods). Notable contributions include Spokas (2010), Budai (2013, IBI), and Lehmann (2015). Persistence is then mostly expressed with a 100-year time horizon, seen as sufficient for policy and technically feasible with the methods used. 

In the 2000–early 2010s, the carbon removal market was still virtually nonexistent or in embryonic form, but biochar started to be recognized with first commercial uses, pilot projects and strong continued research interest. 

IPCC Recognition and Beginning of Removal Certification (mid-2010s–2023) 
The inclusion of a biochar persistence equation in IPCC’s 2019 GHG Inventory Guidelines, albeit as an optional appendix, marked a major milestone. Biochar’s recognition was further supported by its extensive mentions in subsequent IPCC reports on mitigation and negative emissions. The authors of the 2019 IPCC appendix quickly followed with a publication (Woolf et al., 2021) that refined and extended the dataset of biochar decomposition experiments. Biochar permanence remains linked to decomposition modelling and parameters like H/C ratio and carbonization temperature, seen as the primary explanatory variables of persistence, alongside environmental factors like soil temperature. Persistence is still expressed over a 100-year time horizon, albeit IPCC working groups discussed whether it should be expressed on a 1000-year basis. The conclusion was that the methods used were not suited for such long extrapolation, and that 100 years were sufficient at this stage. 

In the mid-2010s–2023, the number of biochar projects increases globally, the first biochar methodologies appear, and the first carbon removal credits are issued. Most methodologies rely on decomposition models over a 100-year time frame – seen as sufficiently conservative – and results across methodologies are generally consistent. Puro’s first biochar methodology (Edition 2019) issued CORCs according to Spokas 2010. With Edition 2022, CORC100+ are issued following Woolf et al. 2021. 

Strong policy and industry interest, research diversification, and market divergence. (2023–present) 
Recent years have brought rapid growth in both market demand and scientific inquiry. Larger-scale biochar projects are now operational globally, numerous other projects are in development, and various policy frameworks and legislation consider biochar activities. Certain stakeholders in the carbon removal market start demanding removals with “millennia” durability. 

Scientific publications on biochar permanence continue to be made. Refinements and discussions on best approaches to decomposition modelling are made (e.g., Rodrigues et al. 2023, Azzi et al., 2024, Li et al. 2024). In parallel, new approaches are put forward, relying on petrographic analysis of biochar (random reflectance) and geological arguments for durability (e.g., Petersen et al. 2023, Drobniak et al. 2024, Sanei et al. 2024). 

Carbon removal standards are prompted to revise their methodologies, while the first EU methodologies are being drafted. Quantification approaches that are included in methodologies start to diverge, in terms of durability claims, equations used, and level of conservatism. The same underlying science is being interpreted in different ways, and established approaches to quantification are perceived as overly conservative. 

In Puro’s latest biochar methodology (Edition 2025, approved on June 12th), biochar carbon removal will be labelled as CORC200+, guaranteeing removal for several centuries. Carbon removal is quantified with a revised decomposition equation, based on open-source science, and tailored for the voluntary carbon market with adequate levels of confidence thresholds. Reporting of novel characterisation approaches, such as random reflectance, is encouraged, but they do not yet affect quantification. 

 

Towards continued scaling and integration of persistence approaches. (next step) 
As already noted in research reports (SLU, 2023), the next step for biochar permanence science is integration. Integration between decomposition modelling and advanced characterisation techniques like random reflectance. Encouragingly, researchers from different disciplines have already started to work together. Furthermore, data presented in scientific conferences does seem to show that there are strong correlations between random reflectance and H/C ratio. Those advances, expected in the next few years, will likely lead all standards to update their quantification approaches, leading to convergence. 

 

Note. We need to acknowledge that the progress described here is built on the tireless work of numerous researchers in different disciplines who have pushed forward biochar’s recognition and enabled commercial scale deployment. 

 

Puro.earth Hosting a Biochar Permanence Workshop 

As interest in permanence grows and new methods emerge, the market must evolve thoughtfully, maintaining both credibility and openness to progress. This summer, Puro.earth is convening fellow carbon standards and scientists in the field of biochar permanence for a collaborative workshop on permanence quantification. 

Workshop goals include: 

Reflecting on current areas of scientific consensus 
Identifying research gaps and opportunities for refinement 
Supporting harmonization where appropriate 
Publishing a transparent summary of best practices and open questions 
At Puro.earth, we view conservative estimates not as a limitation, but as a commitment to clarity and credibility. We’re excited to host this upcoming workshop and collaborate on the next chapter of biochar science—grounded in evidence, built through dialogue, and scaled with integrity. 

 

References: 

Azzi, E. S. and Sundberg, C., Söderqvist, H., Källgren, T., Cederlund, H., Li., H. 2023. Guidelines for estimation of biochar durability - Background report. Swedish University of Agricultural Sciences, SLU Department of Energy and Technology. Report 126 https://doi.org/10.54612/a.lkbuavb9qc 

Azzi, E. S. and Sundberg, C. (2023). On the durability of biochar carbon storage - A clarification statement from researchers. https://biochar.systems/durability-statement/ 

Azzi, E. S., Li, H., Cederlund, H., Karltun, E., & Sundberg, C. (2024). Modelling biochar long-term carbon storage in soil with harmonized analysis of decomposition data. Geoderma, 441, 116761. https://doi.org/10.1016/j.geoderma.2023.116761 

Spokas, K.A. (2010). Review of the stability of biochar in soils: predictability of O:C molar ratios. Carbon Management 1, 289–303. https://doi.org/10.4155/cmt.10.32 

Budai, A., Zimmerman, A. R., Cowie, A. L., Webber. J. B. W., Singh, B. P., Glaser, B., Masiello, C. A., Andersson, D., Shields, F., Lehmann, J., & Camps Arbestain, M. (2013). Biochar Carbon Stability Test Method: An assessment of methods to determine biochar carbon stability. International Biochar Initiative, pp. 1–10. 

Lehmann, J., Abiven, S., Kleber, M., Pan, G., Singh, B.P., Sohi, S., Zimmerman, A. R. (2015). Persistence of biochar in soil. Chapter in: Lehmann, J., & Joseph, S. (Eds.). (2015). Biochar for Environmental Management: Science, Technology and Implementation (2nd ed.). Routledge. https://doi.org/10.4324/9780203762264 

Woolf, D., Lehmann, J., Ogle, S., Kishimoto-Mo, A. W., McConkey, B., & Baldock, J. (2021). Greenhouse Gas Inventory Model for Biochar Additions to Soil. Environmental Science & Technology. https://doi.org/10.1021/acs.est.1c02425 

Rodrigues, L., Budai, A., Elsgaard, L., Hardy, B., Keel, S.G., Mondini, C., Plaza, C., Leifeld, J. (2023). The importance of biochar quality and pyrolysis yield for soil carbon sequestration in practice. European J Soil Science 74, e13396. https://doi.org/10.1111/ejss.13396 

Petersen, H.I., Lassen, L., Rudra, A., Nguyen, L.X., Do, P.T.M., Sanei, H., (2023). Carbon stability and morphotype composition of biochars from feedstocks in the Mekong Delta, Vietnam. International Journal of Coal Geology 271, 104233. https://doi.org/10.1016/j.coal.2023.104233 

Drobniak, A., Mastalerz, M., Knauth, W., Adarkani, O., Dos Santos, T., De Faria, V., Congo, T., Hackley, P., Hatcherian, J., Hower, J., Petersen, H., Reyes, J., & Sanei, H. (2024). Atlas of Microscopic Images of Biochar: Using Reflected Light Microscopy in Biochar Characterization. Indiana Journal of Earth Sciences, 6. https://doi.org/10.14434/ijes.v6i1.37623 

IPCC. (2019). 2019 Refinement to the 2006 IPCC Guidelines for National Greenhouse Gas Inventories. Intergovernmental Panel on Climate Change. https://www.ipcc-nggip.iges.or.jp/public/2019rf/index.html 

Li, H., Azzi, E. S., Sundberg, C., Karltun, E., & Cederlund, H. (2024). Can inert pool models improve predictions of biochar long-term persistence in soils? Geoderma, 452, 117093. https://doi.org/10.1016/j.geoderma.2024.117093 

Sanei, H., Rudra, A., Przyswitt, Z. M. M., Kousted, S., Sindlev, M. B., Zheng, X., Nielsen, S. B., & Petersen, H. I. (2024). Assessing biochar’s permanence: An inertinite benchmark. International Journal of Coal Geology, 281, 104409. https://doi.org/10.1016/j.coal.2023.104409