Many factors will influence carbon stocks on any given area of lands under Article 3.3 or 3.4 (Section 2.3.3). In principle, accounting could focus on only stock changes resulting from human-induced activities, disregarding changes resulting from other causes, or on all stock changes during the commitment period.
It may be possible to use an activity-based approach to identify stock changes resulting only from human-induced activities by developing appropriate factors that represent the impact of these activities on carbon stocks.
Using a land-based approach for ARD activities factoring out the impact on stocks of indirect effects, such as CO2 fertilization and acid rain, would be difficult, if not impossible, because in the absence of the ARD activity the land would be subject to fundamentally different conditions. Therefore, once an area has been designated as ARD land, the practical approach may be to count all stock changes that occur on that land, regardless of the cause.
Counting all stock changes during the commitment period ensures that debits and credits for emissions and removals correspond to actual stock changes. Stock changes during the commitment period will result from many factors. In practice, the fraction of the landscape designated as ARD land is likely to be small, and the nature of ARD implies that these activities are likely to be the dominant factor affecting carbon stocks on these lands. This approach is developed in Chapter 3.
Depending on the nature of additional activities adopted under Article 3.4, in some areas the influence of natural variability and indirect effects (such as CO2 fertilization or climate change) could be the predominant factor affecting carbon stocks during a commitment period (see Section 3.3.3). Consider forest management as an example. If a carbon stock increase is measured in a forest stand that is managed with reduced-impact logging, this practice may not be responsible for all, or even most, of the stock increase. CO2 or nitrogen fertilization could be playing a significant role, or the stock increase could be primarily caused by recovery following disturbance.
Although directly measuring the fraction of the observed stock increase on land attributable to the LULUCF activity may not be possible, the baseline stock increase that is not attributable to the activity could be estimated and subtracted. Appropriate baselines for this purpose might be estimated by using control plots that continue the current management regime or are not subject to active human management.
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