18.104.22.168.4. Comparison of methods
Table 5-9 shows a comparison of the GHG benefits
attributed to the sequestration project illustrated in Figure
5-3. The example assumes the following:
- The project is run for three rotations of 18 years each.
- At the end of each rotation, the carbon stock in the forest reaches 140
t C ha-1.
- Harvesting reduces carbon stock to zero, and the baseline is zero.
Calculations were conducted assuming a minimum required project duration of
55 years [based on the Te of 55 years (Moura-Costa and Wilson, 2000)]
and 100 years [based on the equivalence time of 100 years; see Chapter
2 (Fearnside et al., 2000)]. It is clear from this example that,
depending on the accounting method used, different amounts of carbon benefits
accrue to the project, as is shown by the following results:
- According to the stock change method, this project would receive 140 t C
ha-1 during the sequestration phase of each rotation and would need to return
an equivalent amount after each harvest.
- The average storage calculated for the duration of this project is 84 t
C ha-1 (using the traditional average storage method, without a fixed minimum
project duration), which is reached before the end of the first rotation and
remains the same irrespective of the duration of the project. If a specified
time frame is adopted for the calculation of the average storage (i.e., with
a predetermined denominator in the average storage equation), the GHG benefits
of a project would increase proportionally to the time frame under which the
project is conducted.
- If a minimum project duration of 55 years were required, the equivalence-adjusted
average storage of this project (which is conducted for 54 years) would be
83 t C ha-1, whereas if the minimum time frame required were 100 years, the
equivalence-adjusted average storage would be 45 t C ha-1. Furthermore, if
this project were conducted for only one rotation, the project's benefits
would be lower (see values in parentheses in Table
- Another accounting option (the stock change crediting with ton-year liability
adjustment method) is to use the stock change method to calculate the benefits
of the projects during the sequestration phase and to use ton-years to calculate
the "loss" of benefits when emission takes place. Using this approach, the
calculated GHG benefits of the project at the end of the first rotation would
be 140 t C ha-1 (the same as in the stock change method); when emissions take
place after harvesting, however, the calculated GHG benefits "lost" are either
112 t C ha-1 (if a ton-year equivalence factor Ef
= 0.0182 is chosen, based on Te = 55) or 136 t C ha-1 (if a ton-year
equivalence factor Ef = 0.010 is chosen,
based on Te = 100). The longer the project duration, the smaller the
amount of GHG benefits "lost" after harvesting.
- If the GHG benefits of the project are calculated using the equivalence-factor
yearly crediting method (ton-year accounting), the GHG benefit attributed
to the project would increase gradually as the project is conducted for a
longer time frame. Because this method assumes that the ton-year equivalence
factor reflects the GHG benefit to the atmosphere derived from temporary storage,
no loss of benefits is assumed when emissions take place.