Key category analysis (KCA) is IPCC’s method for deciding which emissions or removals categories to prioritize in preparation of the GHGI. A category is key if, when categories are ordered by magnitude, it is one of the categories contributing cumulatively to 95% of total national emissions or removals, or to 95% cumulatively of the trend in national emissions or removals. KCA is described and in section 5.4 of GPG2003 , and volume 1, Chapter 4 of the 2006GL . Since it is not known at the outset which categories are key, KCA may need to be iterative with the initial ordering undertaken using Tier 1 methods.

REDD+ activities are mostly not recognised categories in the IPCC inventory methodology, but in the case of deforestation, GPG2003 suggests adding up the forest to other land use conversions that contribute to deforestation, and treating deforestation as key if the result is larger than the smallest category considered to be key using the UNFCCC reporting categories. IPCC also provides qualitative criteria for identifying key categories, one of which is that categories for which emissions are being reduced, or removals enhanced, should be treated as key. To the extent that this qualitative criterion applies in the case of REDD+ activities, they could be treated as key, although there has been no COP decision on this.

In applying KCA⁽¹⁾, GPG2003 asks whether particular subcategories defined for this purpose are significant. The subcategories defined for these purposes are for CO₂ are biomass, dead organic matter and soils⁽²⁾. For IPCC, significant subcategories are those which contribute at least 25% to 30% of the emissions or removals in the parent category to which they belong. This does not mean that sub-categories may be omitted, but for subcategories which are not significant in this sense, countries may use Tier 1 methods if country specific data are not available. Identifying key subcategories assists in the allocation of resources to collect country specific data and in addition focuses efforts to reduce uncertainties related to these key subcategories.

Decisions 12/CP.17 and 13/CP.19 say that significant pools and activities should be included in FREL and/or FRLs, and that Parties have some flexibility not to include other pools and activities, considered not to be significant. For reasons of consistency, it is clear that inclusion of pools and activities should be the same in the FREL and/or FRL as for the subsequent emissions and removals estimates from REDD+ activities.

Drawing on a precedent from IPCC usage, significant pools could be taken to be those accounting for 25% to 30% or more of the GHG emissions or removals associated with a REDD+ activity⁽³⁾. The analogy is not exact because IPCC uses the 25% to 30% level to define as significant pools for which default methodologies can be applied, even if the parent category to which they belong is a key category. This is not the same as deciding on potential omission of a pool consistent with decision 12/CP.17 and 13/CP.19 . Another possible (though not necessarily mutually exclusive) way to approach significance, based on a set of rules to help ensure a consistent policy signal, that achievement in emissions reductions or removals enhancement would not be overestimated and to prioritize the most relevant sources. For example:

For deforestation in tropical biomes, the most significant pool will often be biomass, except where forests are growing on organic soils. In the case of other activities, biomass could be regarded initially as the most significant pool and the other pools tested against this working hypothesis using IPCC methods summarized in the MGD, implemented at Tier 1 for test purposes. As an example, where covered by national forest definitions, for planted forests established on drained organic soils, soil organic carbon is very likely to be significant under the rules suggested above because the pool decreases as biomass increases. The expectation would be to include significant pools using country specific data (hence Tier 2), as these become available. Significance can be kept under review as national monitoring systems develop.

As with pools, for activities a possible consideration could be that the REDD+ activity likely to be responsible for the largest GHG emissions or removals be considered the most significant. Activities unlikely to be affected by displacement (causing greater emissions and/or reduced removals) due to action on the most significant activity could be considered not significant relative to the most significant activity. Activities likely to be affected by displacement due to action on the most significant activity would be considered potentially significant, for inclusion at the same time as the most significant activity, or for prioritization in a stepwise approach as better data become available. Evidence for displacement⁽⁴⁾ would include consideration of how action on the most significant activity would affect the drivers of other activities, and hence the emissions and/or removals associated with them. The relationship to proxies may be relevant. Subsequent steps would also allow inclusion of the next largest activities whether or not affected by displacement from activities already included as significant. This process could continue until all activities justified as significant by the Party and considered under the technical assessment process were included.

The relative importance of emissions or removals associated with REDD+ activities may change over time (because of actions taken, evolution of drivers, newly acquired data or improved methods), so significance, where applied, should be reassessed periodically, e.g. as part of a stepwise approach, and in particular when assessing results.