The Time Value of Carbon and Carbon Storage: Clarifying the terms and the policy implications of the debate

The question of whether there is any value to the temporary storage of carbon is fundamental to climate policy design across a number of arenas, including physical carbon discounting in greenhouse gas accounting, the relative value of temporary carbon offsets, and the value of other carbon mitigation efforts that are known to be impermanent, including deferred deforestation. Quantifying the value of temporary carbon storage depends on a number of assumptions about how the incremental impact (or social cost) of a given ton of carbon emissions is expected to change over time. In 2009, a U.S. government interagency working group was established and assigned the responsibility of calculating social cost of carbon estimates to be used in benefit/cost analysis of regulations impacting carbon dioxide emissions. Those estimates were released in March 2010. This working paper explores what those estimates imply about the value of temporary carbon storage, as well as the implications of those temporary storage values for several critical policy design questions relating to greenhouse gas accounting and biological offsets. This analysis suggests, for instance, that appropriate physical carbon discount rates for carbon accounting may be even lower than the social discount rates often used in intergenerational analyses. In the context of agricultural offsets, the social cost of carbon estimates are used to establish a definition of equivalence between permanent and temporary offsets; equivalence ratios are derived that vary between ~2 and 30, depending on the discount rate used and the length of the temporary offset contract period.