Integrating time in definitions of carbon sequestration and greenhouse gas removals and reversals.

The permanence or temporary nature of carbon removals from the atmosphere into natural systems has been a major topic of debate in the design of accounting methodologies for climate policy. These topics are currently being discussed in the preparation of a new mechanism established by the Paris Agreement. Emerging concepts from the field of ecosystem ecology could provide the key metrics needed to address the problems of permanence and reversibility of removals in carbon accounting. I show here how the concept of compartmental system provides the level of generality needed to address the temporal nature of removals in carbon accounting, with clearly defined metrics for assessing climate benefits of temporary storage. I also show how the variable time can be incorporated in the definitions of carbon sequestration and removals in a way that would encourage the development of climate actions that would keep carbon out of the atmosphere for long periods of time. In contrast to traditional 'stock change' or 'tonne-year' accounting methods, the approach proposed here takes explicit consideration of the time that carbon remains stored in a natural system and does not contribute to warming in the atmosphere. The proposed methodology may resolve current issues related to the definition of baselines, permanence and quantification of temporary storage and reversals.