2.3.6.3. Equivalence Time and Ton-Years

An alternative approach is to compare activities that sequester (or release) carbon for different lengths of time by using an accounting convention or equivalency factor. The basic policy question that must be answered for any such system is how long carbon must be sequestered to be considered equivalent to "permanent" emission avoidance. (Article 3.3 of the Kyoto Protocol states that accounting should be based on verifiable changes in stocks in each commitment period-apparently precluding an equivalency factor approach). Several authors have analyzed the benefits of sequestration projects being accounted for on a ton-year basis rather than by requiring "permanent" sequestration. Ton-year accounting (Fearnside, 1995, 1997; Moura-Costa, 1996; Bird, 1997; Chomitz, 1998a; Dobes et al., 1998; Tipper and de Jong, 1998; Moura-Costa and Wilson, 1999) would allow comparisons between avoided fossil fuel emissions and sequestration activities as well as among sequestration activities of different duration. Under a ton-year system, credit would be given for the number of tons of carbon held out of the atmosphere for a given number of years. A ton-year accounting system would provide a basis for temporary sequestration or delayed deforestation to be credited; the mitigation benefit from a given patch of land is greater the longer the carbon remains in place-which would be reflected in the credit earned.

As long as the policy time horizon is finite or a non-zero discount rate is applied to determine the present value of future emissions/ removals, even short-term sequestration will have some value. The explanation of this proposition is made clearer by considering the converse case: emission of 1 t CO2 followed 20 years later by removal of 1 t CO2. Although the net emission over the entire period is zero, there clearly has been an effect on the atmosphere. A ton-year equivalency factor can be used to determine the relative climate effect of different patterns of emissions and removals over time. For a given pattern, this factor will be a function of the time horizon and discount rate selected.

Alternative methodologies have been proposed to generate this equivalence time parameter. Tipper and de Jong (1998), for example, base their calculations on the difference between current atmospheric concentrations and the pre-industrial "equilibrium" concentration of CO2 to derive a carbon storage period (Te) of 42-50 years following initial sequestration. Similar ranges have been proposed by Bird (1997; Te = 60 years) and Chomitz (1998b; Te = 50 years). Dobes et al. (1998) analyzed the effect of storage as a delay in emissions and calculated Te = 150 years.

A similar problem has been addressed through the use of GWPs to compare emissions of GHGs that have different residence times in the atmosphere (as well as different radiative forcing per molecule). Although this concept has limitations (IPCC, 1996; Smith and Wigley, 2000), it has been adopted for use in the Kyoto Protocol to account for the total emissions of covered GHGs on a CO2-equivalent basis. Absolute Global Warming Potentials (AGWPs) are calculated by integrating the total radiative forcing of an emissions pulse over a 100-year time horizon with no discounting. Relative GWPs are the ratio of this integral for a given GHG to that of CO2, which serves as the reference gas. This approach could be applied to compare carbon sequestration projects of different lengths, although there is no requirement in the Protocol to use the same conventions in this context. The reference is "permanent" (more than 100 years) removal (or emission) of 1 t CO2. Based on the carbon cycle model used to calculate GWPs in the Second Assessment Report (SAR) (Joos et al., 1996), this approach results in a reduction (or increase, in the case of an emission) in the cumulative CO2-C loading of 46 ton-years relative to a reference scenario with no emission or uptake.⁴

Figure 2-5: Perturbation of atmospheric concentrations from removal of CO2 in year zero followed by emission of the same quantity of CO2 in year 46. The initial change in concentration relaxes toward the unperturbed state, based on a reduced form of the carbon cycle model used to calculate Global Warming Potentials in the Second Assessment Report (Joos et al., 1996). The net effect on the atmosphere over the 100-yr time horizon shown is the difference between the areas under the two curves, which is equal to the integral of the removal curve from year 54 to year 100.

A carbon sequestration project with a duration of 46 years, for example, can be analyzed as a removal of 1 t CO2 in year zero followed by emission of 1 t CO2 in year 46. As Figure 2-5 illustrates, the net reduction in atmospheric CO2 burden from this project is the difference between the integrated effect of these two events (over a 100-year time horizon, for consistency with the GWP approach). The result is a reduction in atmospheric burden of 17 ton-years (the difference between the integrals of the two curves within the 100-year time horizon: 46 ton-years - 29 ton-years), or 37 percent of the effect of a "permanent" removal (or avoided emission). Note that the relative value of such a project would be higher with a shorter time horizon (100 percent for a time horizon of less than 46 years) and lower for a longer time horizon (6 percent for a time horizon of 500 years) (Fearnside et al., 1999).

Figure 2-6: Alternative methods of crediting carbon sequestration.

The foregoing calculations consider the difference between the mitigation and baseline scenarios by comparing the integrals of the atmospheric load of carbon over the time horizon. This approach focuses on carbon in the atmosphere for computing the benefits of LULUCF mitigation projects-as contrasted to some analyses that focus attention on carbon in trees (e.g., Moura-Costa and Wilson, 1999). The distinction between carbon in trees and carbon in the atmosphere is important because carbon in the atmosphere is subject to removal through natural processes that transfer it to sinks such as oceans and the biosphere, whereas carbon in trees is assumed to remain fixed. If carbon release is delayed because it is held in trees, the benefit is represented in this approach by the area of the tail of the second curve that is pushed beyond the end of the time horizon as a result of the delay in emissions (see Figure 2-6)-not by the larger area that would be described by a rectangle representing carbon stock in plantation biomass over the life of a mitigation project. In the approach proposed by Moura-Costa and Wilson (1999), an "equivalence time" is calculated as the point at which the area of the rectangle representing biomass carbon is equal to the area under the atmospheric carbon decay curve over the 100-year time horizon.

If the ton-year approach is adopted, incremental credit can be awarded for each year that carbon stocks remain sequestered. The cumulative award of credit would equal the credit from a "permanent" emission reduction of the same magnitude if the stocks remained intact for 100 years. If the stocks were released at any time prior to the 100-year time horizon, only the appropriate amount of partial credit would have been awarded (see Table 2-6).

Ton-year accounting requires some data that are not required by systems that are based solely on "snapshots" of carbon stocks at the beginning and end of each commitment period. However, for most LULUCF activities, such as plantation silviculture, the carbon stocks in intermediate years can be inferred from the snapshot data, supplemented with records of harvest and planting dates that should be readily available.

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