OPINION: Rethinking fNRB to improve cookstove credit accuracy
Chris McKinney is chief commercial officer at Kenya-based cleaner cookstoves manufacturer and project developer Burn.
The carbon sector is awash with acronyms, but "fNRB" demands particular attention.
The fraction of non-renewable biomass (fNRB), is a key driver in the accuracy of cookstove carbon crediting.
With 2.5 billion people in the global south lacking access to life-saving improved stoves, getting it right really matters.
This week brings fNRB into the spotlight, as proposed new defaults for this widely used (and misunderstood) variable, are under consultation until Thursday August 1.
What is fNRB?
Wood can be a sustainable cooking fuel only if the surrounding landscape can regrow and replace the biomass burned.
But if more wood is harvested than a forest can produce, then biomass consumption is considered non-renewable.
Across the Global South we see high rates of forest cover loss, with informal charcoal production driving deforestation and degradation.
The fNRB attempts to quantify what percentage of the wood burned for cooking is unsustainable.
Over the last 18 months, fNRB has become a point of concern in cookstove carbon crediting.
Commentators have pointed to the wide gap between the old CDM defaults [1] (80-90% fNRB), and a study from 2015 which gave a global default of 30% fNRB [2].
Now new defaults generated using the MoFUSS statistical modelling tool suggest an fNRB of 30-50% fNRB [3].
The difference between the old and new defaults has a simple explanation. The definition of fNRB has actually changed over time and this change has far-reaching implications.
A marginal definition for fNRB?
The latest fNRB numbers consider the renewability of all wood fuel harvesting nationwide (a gross definition).
But the original CDM definition was to consider the renewability of the change in wood fuel harvesting (a marginal definition).
In economics, the term "marginal" refers to the impact of one added or subtracted unit.
For example, "marginal cost" is the additional cost incurred from producing one more unit of a good or service.
The study of marginality should also be applied to wood fuel harvesting.
Imagine a family that lives beside a forest which sustainably grows 2 tonnes of biomass each year.
If the family goes from harvesting 4 tonnes a year at baseline, to 3 tonnes in the project scenario, but the forest keeps growing 2 tonnes per year, then it makes sense that the tonne of wood saved is from the non-renewable portion of the harvest.
The current application of fNRB assumes that every tonne of wood saved is the same proportion renewable and non-renewable.
A marginal definition of fNRB recognises that savings in wood consumption come first from the unsustainable portion of the harvest.
A marginal definition would bring cookstove methodologies in line with the emission reduction approach in energy efficiency projects (e.g. AMS II.C).
In this instance, the methodology uses a marginal grid emission factor.
This means it focuses on what generated the saved units of electricity, rather than the emission factor of the grid as a whole.
This approach considers that the marginal unit of electricity often originates from sources that are more easily switched off (e.g., coal-fired power stations), which typically have a different emission profile compared to the overall grid, which might be balanced with renewable energy sources.
There is clear expert consensus that a marginal method is the only accurate assessment of climate impact, because only a small set of generators change output (and thus emissions) due to a specific change in total energy demand.
We argue that the same dynamics apply to wood fuel harvesting, and a growing number of experts agree that further research is needed into marginality and fNRB.
What does this mean moving forward?
Last month, the UNFCCC published draft national fNRB defaults based on the latest developments in statistical modelling.
The good news is that the MoFUSS tool used to create these numbers can also generate marginal fNRB defaults.
MoFuSS was designed to compare baseline and intervention scenarios.
This means we can estimate the non-renewability of the reduction in woodfuel harvesting, as well as non-renewability of the harvest in the landscape overall.
In fact, preliminary indications suggest that the first tonnes of reduced harvest are in general much more non-renewable than the harvest as a whole.
Further research is needed, and we therefore call for more funding to create marginal MoFUSS defaults, and more time for a thorough exploration of fNRB as a marginal variable.
The CDM Methodology Panel consultation closes this week, on August 1, and we encourage all parties to respond.
Getting this right is important, making this week a key moment for all of us working to improve the accuracy of clean cooking carbon crediting.
[1] CDM: https://cdm.unfccc.int/DNA/fNRB/index.html
[2] Bailis et al., (2015), https://www.nature.com/articles/nclimate2491
[3] Bailis et al. (2024), https://cdm.unfccc.int/public_inputs/2024/202406/index.html
