Keeping it 100 – Permanence in Carbon Offset Programs

July 26, 2022

By Jennifer Weiss, Vice President, Communications and Business Outreach, Climate Action Reserve

(originally posted on

Permanence is a key tenet of carbon offset programs. In order for greenhouse gas (GHG) emissions reductions to earn offset credits and have value in the carbon markets, the GHGs must be permanently reduced or sequestered. Permanence is defined as providing lasting benefits to the environment. For high quality offsets, permanence is defined as at least 100 years of GHG reduction or sequestration.

GHGs have different heat-absorbing abilities and stay in the atmosphere for different lengths of time. Global Warming Potential (GWP) is a value that compares the heat-absorbing ability of GHGs relative to that of carbon dioxide. The IPCC calculates GWPs for GHG inventory purposes over several timeframes, with most policy applications using a 100-yr timeframe where values can range from 25 – 22,800 times that of carbon dioxide. High quality offset standards calculate carbon crediting utilizing a 100-year GWP. Therefore, allowing any program to subsequently protect the sequestered carbon for something less than 100 years is equivalent to awarding 100 years’ worth of climate benefits without a corresponding requirement to actually deliver 100 years’ worth of climate benefits.

Ensuring 100-year permanence in GHG reduction and sequestration projects

For GHG reduction and/or destruction projects, permanence is achieved through the installation and operation of technologies that enable the collection and destruction of GHGs. For example, methane collection and combustion systems at landfills consist of wells, pipes, blowers, caps and other technologies that enable or enhance the collection of landfill gas and convey it to a destruction technology, such as flares, turbines, reciprocating engines, fuel cells, boilers, heaters, or kilns, that permanently destroys the captured methane.

For sequestration-based GHG reduction projects, 100-year permanence is achieved by ensuring that the carbon associated with credited GHG projects remains stored for at least 100 years. Examples of nature-based sequestration projects include forest offset projects, in which trees are planted and/or managed to increase absorption of CO2 from the atmosphere and carbon storage in their biomass, and soil enrichment projects, in which soils gain carbon through the decomposition of newly added carbononaceous materials.

Nature-based sequestration projects face the potential for reversals if stored carbon is released back to the atmosphere. Unavoidable reversals result from uncontrollable natural agents such as fire, insects, disease outbreaks, and wind. Avoidable reversals result from controllable agents or human activities such as land conversion and over-harvesting.

In order to ensure the 100-year permanence of nature-based sequestration projects, the Reserve employs three mechanisms: (1) requiring projects to monitor and verify onsite carbon stocks for a period of 100 years following the issuance of any offsets. For example, if CRTs are issued to a forest project in year 99 following its start date, monitoring and verification activities must be maintained until year 199. (2) Requiring all Project Operators to sign a Project Implementation Agreement with the Reserve, which obligates Project Operators to retire CRTs to compensate for reversals of GHG reductions and removals. (3) Requiring contribution to a Buffer Pool to provide insurance against reversals of GHG reductions and removals due to unavoidable causes.

Two accounting approaches to 100-year permanence: tonne-tonne accounting and tonne-year accounting

The Reserve employs two accounting approaches to achieving 100-year permanence:

  1. Use tonne-tonne accounting (TTA) for any nature-based offset projects willing to commit to a 100-year permanence requirement. Under tonne-tonne accounting, a credit is issued for every additional tonne of GHG emissions that is sequestered permanently (defined as a period of 100 years) with required long-term monitoring during that timeframe.

    The common critique of this approach is that it is unrealistic and cannot be implemented at the scale needed, but 85% of the offset credits used in the California compliance market come from forestry projects that require a 100-year commitment, demonstrating that this approach is both technically correct and feasible at scale.

  2. Use tonne-year accounting (TYA) for any nature-based offset projects that utilize a permanence requirement of less than 100 years. TYA accounting essentially awards a pro-rated share of carbon credits for each successive year that carbon is sequestered. To over-simplify for illustrative purposes, under tonne-year accounting when employing 100-year GWPs for calculating environmental benefits, for each incremental year of successful carbon sequestration, the project would receive 1/100th of the environmental value. TYA assumes that for each year a tonne remains sequestered, 1/100th of the 100-year climate benefit is achieved and credits are awarded at a rate of one percent per tonne per year for the crediting period.

Whether employing tonne-tonne accounting or tonne-year accounting, GHG reductions and sequestration must rely on a 100-year timeframe in order for the market to be confident permanence is appropriately being safeguarded and for credibility when utilizing 100-year GWPs to calculate carbon crediting.

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