Rating by vintage: why a project rating can change over time

Tuesday, 08 Aug 2023

Calyx Global sometimes gives multiple ratings for a project based on the vintage of the credits rather than one rating for the entire life of the project. The vintage refers to the year when the claimed emission reductions or removals occurred. Conditions can change during a project's lifetime, and giving ratings for different time periods allows Calyx Global to provide our customers with more accurate evaluations of the project's greenhouse gas (GHG) integrity over time. This is also true for Sustainable Development Goal (SDG) contributions, where a project's impact can change over time. Different project categories can have different concerns about changes within a project's lifetime; we provide some examples below.

Renewable energy

Renewable energy is experiencing rapid global growth. As a result, the circumstances surrounding renewable energy projects can change within the project's lifetime. Some projects have a lifespan of 30 years. During this time, many host countries have witnessed significant development in renewable energy. Factors such as new legislation, incentives, and government support for expanding renewable energy can change the overall composition of an electrical grid. 

Typically, later vintages of a renewable energy project have a higher risk. This occurs if a project becomes common practice or if the grid emission factor (GEF) changes significantly but the project does not update it. New legislation could also make a project more financially attractive. If the national government implements an emissions trading or cap-and-trade system, or other mandatory policies that affect renewable energy development or usage, the project's risk of overlapping claims could change.

Landfill gas

Most landfill gas (LFG) projects have 10-year crediting periods with a total lifetime of 20 to 30 years. New regulations, financial incentives and changes in common practice or the project’s overall function can affect a project’s additionality. Yet, additionality is often not re-assessed at the beginning of each crediting period. This absence of reassessment can put additionality at risk if such changes take place. 

This situation is particularly noticeable for landfill gas projects in Turkey. The existing feed-in tariff saw a significant increase in 2010, potentially making some projects, which had previously depended on carbon revenue, financially viable without that revenue. If the project did not conduct an updated investment analysis at the crediting period renewal, this would result in a higher risk for the subsequent crediting periods.

Another case that results in separate ratings for vintages is when a landfill gas project starts as a flaring-only project, but later begins utilizing captured LFG to produce electricity or heat. Since there is no remuneration for the destruction of LFG, the initial flaring-only vintages of the project have a lower risk of non-additionality. However, monetizing LFG by producing electricity has the potential to be profitable, so the later vintages may have an increased risk of non-additionality.

Forests & land 

The forests and land sector includes various project types that share common risks related to changes in non-additionality, as well as non-permanence as projects age. Additionality tends to improve over time, while the risk of reversals often increases.

Concerning the risk of non-permanence, one important element is the length of time that the project is committed to monitoring for reversals. Presumably, any such reversals would be compensated (by the project owner or through the use of an insurance mechanism, such as a pooled buffer reserve). The lifetime of a project is often the period for this monitoring, with only a couple of exceptions – for example, Climate Action Reserve (CAR) represents best practice and requires monitoring for 100 years after the last vintage. In most cases, the project only monitors during its lifetime. This duration varies depending on the project type, standard/protocol or choice made by the project developer. Some projects can have a duration of up to 100 years. The shortest duration (Architecture for REDD+ Transactions’s requirement for Jurisdictional REDD) is 5 years. Most REDD projects are 30 years, while afforestation and reforestation (AR) projects vary and can be up to 100 years. 

In the Calyx Rating system, the risk of non-permanence often increases over time. This is because only CAR (and California’s compliance trading system) require a certain number of years post-vintage to be monitored. For other standards, the number of years that any particular vintage credit is monitored will decline over time. In the example of a 30-year REDD project, the 25th vintage year will only be monitored for an additional 5 years, presenting a high risk. Our risk rating for non-permanence increases over time as the project’s period of commitment to monitoring and compensation for reversals declines. 

Unlike other project types where additionality is at greater risk in later years (due to technology advancements and implementation), forest and land projects tend to see higher risks in the earlier years. This is often because projects fail to demonstrate prior consideration of carbon revenues. Among forest projects rated by Calyx Global, there have been several cases where it does not appear that carbon revenues played a role in early vintages. 

In some cases, projects were fully funded by grants or subsidies and only later became dependent on carbon revenues. In such cases, we expect the crediting start date to begin at the time in which the project considered carbon revenues as critical for the continuance of its activities. Verra’s Verified Carbon Standard (VCS) has allowed a long period from crediting start to validation – up to five years – and many projects have “back-dated” to claim up to five years of credits during a period where carbon finance was not considered. Other projects, by contrast, select their “crediting start date” by choosing a year that coincides with the consideration of, and need for, carbon revenues. This approach leads to a higher additionality score in our framework.


All of Calyx Global’s GHG integrity risk factors may change over time for cookstove projects. However, the risks of non-additionality, overestimated baseline and project emissions are the most likely to change during a cookstove project’s lifetime.

During the project’s life, new legislation, incentives, social and macroeconomic conditions can impact whether improved cookstoves or clean cooking devices become common practice and/or require carbon finance. This impacts the additionality of cookstove projects. 

Changing conditions may also impact the cookstove technologies used by the targeted population and, thus, the consumption of biomass fuel that determines the baseline. Project emissions may also change if project developers choose to deploy new or different types of stoves.

Finally, the fraction of non-renewable biomass can also change based on the pace of degradation in relevant forests caused by wood fuel demand or other reasons for wood extraction. 

Issues that impact the GHG rating for all sectors

All project types can experience changing risks of overlapping claims. For example, cookstove or biodigester projects may be claiming the same emission reductions from biomass conservation as REDD projects. New projects may appear that increase this risk. Similarly, renewable energy projects may face an increase in overlapping claims if the government sets new mandatory mitigation targets for energy in the country. 

Project ratings can also change if, for example, we see penetration rates of a technology rising without a need for carbon revenues. In this case, all ongoing projects in that country or region would have their ratings reviewed. For some technologies, such as renewable energy, we check these issues on an annual basis.

Sustainable Development Goals

Calyx Global ensures that verified contributions to SDGs correspond to the time periods of each vintage. We evaluate all relevant monitoring and verification cycles of the project and assess the durability and validity of SDG contributions for each vintage year.

In some cases, the overall SDG rating differs between vintages. For example, a more recent monitoring cycle may offer more or less evidence of a particular SDG contribution. Or, we may note that the SDG certification (i.e., CCB, Gs4GG) does not apply to a particular vintage, in which case we mark those vintage years as N/A.

About the author

Calyx Global

This article includes insights and input from multiple experts in Calyx Global.