Why permanence matters more than buyers think
A tonne of CO2 stored for one year and then re-emitted does not solve climate change. The atmosphere does not care whether the carbon was 'in a forest' for that year. What matters is total residence time outside the atmosphere. Permanence — the expected duration that the avoided or removed carbon stays out of the atmosphere — is the single quality dimension that determines whether a credit delivers what it promises.
How permanence is categorised
- Short-term (under 25 years): cookstove projects, some soil carbon, short-rotation agroforestry. Mostly avoidance, vulnerable to reversal.
- Medium-term (25-100 years): forest carbon (ARR, REDD+, IFM), most blue carbon projects, mangrove restoration. Vulnerable to fire, drought, deforestation.
- Long-term (100-1000 years): biochar in soil, mineralisation, ocean alkalinity enhancement. Engineered storage with documented stability.
- Geological (1000+ years): direct air capture with geological storage, bio-oil sequestration, BECCS with CO2 injection. Permanent for practical purposes.
Buffer pools — the industry's permanence insurance
Verra, Gold Standard and ACR all require nature-based projects to contribute a portion of issued credits — typically 10-20% — to a shared buffer pool. If a project suffers a reversal (fire, illegal logging), credits are withdrawn from the buffer to compensate. A buffer ratio of 20%+ is generally considered robust. Below 10% is concerning.
Buffer pools work for moderate, independent reversal events. They do not adequately protect against correlated systemic risks — for example, if a regional drought causes simultaneous reversals across multiple projects in the same buffer pool. This is one reason why diversifying across geographies and project types matters even for highly buffered portfolios.
How ClimSen scores permanence
The ClimSen Premium Score awards up to 15 points for permanence, weighted by the combination of expected storage duration and buffer pool adequacy. A biochar project with 100-year minimum permanence and engineered verification typically scores 13-15. A forest project with 40-year permanence and a 20% buffer scores 10-12. Projects with weak permanence (under 25 years) or insufficient buffer (under 10%) score below 7 and rarely make it onto the marketplace.