GHG accounting (deep dive)

Biochar, greenhouse gas accounting, and climate change mitigation

Biochar is one of the few carbon removal strategies that can be deployed today, using existing infrastructure, and yield measurable benefits in soil and climate systems. Its role in greenhouse gas (GHG) mitigation rests on two pillars: reducing emissions and stabilizing carbon. For GHG accounting frameworks, the challenge is to quantify these effects in consistent, verifiable terms that hold up under scrutiny and can be monetized in carbon markets or integrated into mitigation policy.

At its core, biochar locks carbon into a stable form. Pyrolysis converts biomass into a structure that resists microbial decay. When added to soil, this carbon can persist for centuries. That makes biochar a tool for net removal of CO₂ from the atmosphere—particularly when the feedstock is waste biomass that would otherwise decompose and emit CO₂, methane, or nitrous oxide. The longer that carbon stays out of circulation, the stronger the mitigation benefit.

But carbon stabilization is only part of the picture. Biochar can also reduce GHG emissions from agricultural systems. When applied to soils, it lowers emissions of nitrous oxide and methane by improving aeration, changing microbial pathways, or reducing the availability of reactive nitrogen species. These are indirect effects, but they can add up—especially in high-emission systems like rice paddies, manure-rich soils, or intensively fertilized crops.

In accounting terms, mitigation through biochar falls into three categories:

1. Avoided emissions from feedstock disposal. Converting manure, green waste, or crop residues into biochar prevents emissions that would occur if the material decomposed or was burned.

2. Sequestered carbon in the biochar itself, which is added to soil or embedded in long-lived materials.

3. Reduced emissions from agricultural soil processes due to changes in nitrogen cycling and microbial activity.

Each of these must be estimated separately and aggregated to get a total climate impact. For markets or regulatory systems, this information feeds into life cycle assessments (LCAs), carbon offset methodologies, or national inventory reports.

The baseline is critical. To claim a mitigation benefit, one must demonstrate that biochar leads to lower emissions than the alternative. If crop residues are already being composted, the baseline emissions might be low. But if those residues are typically burned or left to decay anaerobically, converting them to biochar offers a clear advantage. For manures and organic wastes, avoided methane and nitrous oxide emissions can be substantial—especially in regions where waste is landfilled or stored in wet conditions.

Accounting for sequestration requires estimating how much of the carbon in biochar remains in the soil over time. Models based on decay curves, field studies, and stable isotope analysis suggest that 50–90% of biochar carbon is stable over 100+ years, depending on feedstock and production temperature. This fraction, multiplied by the biochar application rate and adjusted for carbon content, gives a first-order estimate of long-term storage.

Quantifying reductions in soil-based emissions is more complex. Nitrous oxide emissions vary with moisture, temperature, fertilizer use, and microbial dynamics. Biochar’s effect depends on soil type, crop system, and climate. Average reductions reported in meta-analyses range from 10% to 50%, but site-specific measurements are needed for rigorous accounting. Similarly, methane reductions are most pronounced in flooded or poorly aerated soils; in upland systems, the effect may be negligible or even reversed.

In carbon markets, mitigation benefits must be real, additional, measurable, and verifiable. That means projects need robust monitoring, reporting, and verification (MRV) systems. For biochar, this includes documenting feedstock source, pyrolysis conditions, carbon content, application method, and site characteristics. It also requires estimating decay rates, avoided emissions, and any co-benefits or leakage. These data feed into methodologies like those developed by Verra, Gold Standard, or other offset registries.

Leakage and permanence are important considerations. If biochar is made from biomass that would have otherwise been used elsewhere—such as animal bedding or compost—it may displace emissions rather than reduce them. If land use changes to supply feedstock, the net benefit may be eroded. And if biochar is later disturbed, oxidized, or removed, the carbon credit may no longer be valid. Projects must address these risks through conservative assumptions, buffer pools, or site management plans.

The economic value of biochar as a mitigation tool depends on both its cost and its creditability. Biochar systems vary widely in economics depending on scale, feedstock availability, equipment, and market access. If biochar can be sold for soil improvement, filtration, or construction, its climate value is a co-benefit. If the primary revenue is from carbon credits, price and verification costs become central. In either case, the accounting must be defensible.

Several protocols are now in place to support biochar carbon credits. These set rules for what counts as permanent storage, how to test and document biochar quality, and how to model long-term carbon behavior. Some allow for crediting reductions in methane or nitrous oxide as well, though this typically requires direct measurement or highly conservative assumptions. As data improves, these protocols will become more precise and accessible.

Beyond offset markets, biochar can support national and international mitigation goals. Under the IPCC framework, biochar may be reported in land use, agriculture, or waste sectors, depending on feedstock and application. Countries with net-zero or negative emissions targets may integrate biochar into their climate strategies, either through subsidies, carbon taxes, or recognition in national inventories.

As the need for carbon removals increases, biochar is likely to play a growing role. It doesn’t require new land, competes minimally with food production, and offers co-benefits in soil and waste systems. Its main challenge is scale. Producing and applying enough high-quality biochar to make a dent in global carbon budgets will require investment, infrastructure, and coordination. But the potential is clear: durable carbon storage, emission reductions, and improved resilience in managed ecosystems.

In conclusion, biochar is both a carbon sink and an emission reducer. Accounting for its impact means understanding feedstocks, pyrolysis, soil processes, and carbon markets. With proper measurement and verification, biochar systems can deliver real, additional, and lasting climate benefits—making them a viable component of mitigation portfolios from farms to national inventories.