The recent release of the draft “Reversal Risk Assessment” Tool under the PACM has reignited an old debate: what project types should be subject to permanence requirements?
A line of argument gaining traction is that project types either produce “flow-based” or “stock-based” claims, and only the latter need permanence requirements. While this line of argument sounds intuitive at a surface level, it points the debate in the wrong direction.
The argument rests on three misconceptions. The following sections examine each in turn:
They are not. Flow-based and stock-based are different accounting approaches for the same reality. Whether you measure changes in a carbon stock or compare emissions and removals over time, you arrive at the same emission reductions, removals, and reversals. When you measure stocks, you compare actual and baseline carbon stored in the reservoir; when you measure flows, you compare actual and baseline emissions and removals into and out of it (the ‘flows’). Both methods quantify the same change.
What matters for permanence is whether a project affects a carbon reservoir. The most common project types do: Avoided deforestation, improved forest management (IFM) and afforestation all either conserve or enhance a biospheric reservoir, namely trees, soils, shrubs and other forms of biomass. Projects that reduce fossil fuel use, such as renewable energy or energy efficiency, also conserve a reservoir: the carbon in the fossil fuels that stay in the ground rather than being burned. Which reservoirs cookstove projects affect depends on the fuel they displace: a stove that reduces firewood consumption conserves a biomass reservoir, while one that displaces a fossil fuel conserves the fossil reservoir instead. This FAO report explains all of this in detail.
In every case a reservoir is being conserved, and carbon held in a reservoir can always, in principle, be released again. What differs across project types is not whether that risk exists, but how large it is. That is where the next misconception comes in.
It does not. As explained above, almost every project type carries some reversal risk, because almost every one conserves a reservoir; what differs is the degree. The case of renewable energy illustrates this well: Renewable energy projects reduce emissions by keeping fossil carbon in the ground relative to the baseline, so a fossil fuel reservoir is being conserved. Yet renewable energy projects are not subject to non-permanence requirements, because unextracted fossil carbon is unlikely to be released into the atmosphere at random and the energy demand is met elsewhere. This is different for projects conserving or enhancing forests, as forests can burn, be cleared, or be converted. Thus, reversal risk depends on how vulnerable a reservoir is.
The same logic applies to cookstove projects; therefore, their reversal risk depends on what type of fuel they displace. A project that reduces the use of firewood conserves carbon in a forest, and that carbon can later be lost, for example due to fire or storms. In this case, the household’s reduced wood fuel use remains real; but the forest stock the credit relied on does not exist anymore. In contrast, a stove that displaces a fossil fuel prevents the release of carbon from a fossil reservoir, therefore carrying a much smaller reversal risk. This is exactly the line the draft Article 6.4 tool draws, imposing buffer requirements on projects that reduce non-renewable biomass but not on those that displace fossil fuels.
Every carbon credit asserts the same thing: one ton of CO₂e avoided, reduced or removed. In practice, there are no flow claims or stock claims underlying carbon credits. If a carbon credit based on a cookstove project is used to offset fossil fuel emissions and the underlying emission reduction later reverses, there are now more net emissions in the atmosphere, independent of whether the cookstove project used flow-based or stock-based accounting.
A more meaningful distinction concerns the use of carbon credits for either offsetting claims or contribution claims. If carbon credits are used for offsetting claims, non-permanence requirements are indispensable, because carbon credits compensate for emissions elsewhere. As contribution claims do not imply any form of compensation, one could argue that non-permanence requirements are less crucial. However, this distinction depends on the use of carbon credits, not the accounting as stocks or flows.
Ultimately, the permanence debate should focus on physical reality rather than accounting terminology. Stocks and flows are simply two ways of measuring the same changes in carbon reservoirs. The relevant question is not whether a project is “stock-based” or “flow-based,” but whether the reservoir a project conserves can later release carbon to the atmosphere, and how likely that release is. Framing permanence requirements around reservoir-specific reversal risk leads to a more coherent and scientifically grounded approach.