Additionality Testing

Testing any given project or tonne of emissions reductions or carbon sequestration to determine whether they are “additional” is hard, and there’s one very simple reason for that. There is no way to empirically validate whether something actually was additional.

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What this has led to in carbon offset markets has been a whole series of ways of trying to test for additionality, including:

  • Environmental additionality tests
  • Financial additionality tests
  • Regulatory additionality tests, and
  • Barriers additionality tests

From the standpoint of the environmental integrity of offset markets, however, all “additionality tests” are nothing more than proxies for trying to determine whether a given project and/or tons of emissions reductions or sequestration can be traced back to the incentives created by the carbon market itself. If so, it’s additional; if not, then it’s not additional.


One reason that additionality testing is complicated is because there are so many things that drive (or impede) most projects resulting in emissions reductions or carbon sequestration, making it hard to determine whether the existence of a carbon market was important enough to justify awarding it offset status.

What we’re really talking about when it comes to additionality testing is determining how confident we can be in the hypothesis that a particular tonne of emissions reductions or sequestration is additional. Although you’ll almost never heard it discussed this way, carbon offset additionality testing is simply an example of hypothesis testing, which requires the balancing of “false positives” and “false negatives,” given that they are inversely correlated.** See Hypothesis Testing.

To dig deeper into this critical topic:

The fact that additionality testing is simply an example of hypothesis testing, combined with the fact that this is so rarely recognized and addressed in the design and implementation of carbon offsets, is the biggest explanation for why the environmental integrity of offset markets has been so frequently challenged.

The failure to even ask how false positives and negatives are balanced in any given additionality testing protocol is perhaps the single best explanation of the environmental integrity problems of carbon markets.

Lastly with respect to additionality testing, it is common to hear it assumed that the verification process associated with carbon offsets results in the validation of a project’s additionality (and the additionality of its associated tons). This reflects a misunderstanding of the verification process, which cannot verify the additionality of offsets being “verified.” Verifiers do not develop additionality tests and protocols, or evaluate the hypothesis that tons are additional. Additionality involves building a counter-factual case that can rarely if ever be empirically verified.**