Methodological Approaches To Identify Key Categories

It is good practice for each country to identify its national key categories in a systematic and objective manner, by performing a quantitative analysis of the relationships between the level and the trend of each category's emissions and removals and total national emissions and removals.

Two Approaches for performing the key category analysis have been developed. Both Approaches identify key categories in terms of their contribution to the absolute level of national emissions and removals and to the trend of emissions and removals.

In Approach 1, key categories are identified using a pre-determined cumulative emissions threshold. Key categories are those that, when summed together in descending order of magnitude, add up to 95 percent of the total level4. The method is described in more detail in Section 4.3.1, Approach 1 to identify key categories.

4 The pre-determined threshold has been determined based on an evaluation of several inventories, and is aimed at establishing a general level where 90% of inventory uncertainty will be covered by key categories.

Approach 2 to identify key categories can be used by inventory compilers, if category uncertainties or parameter uncertainties are available. Under Approach 2, categories are sorted according to their contribution to uncertainty. This approach is described in more detail in Section 4.3.2, Approach 2 to identify key categories. Results of Approach 2 are additional to Approach 1. If both the Approach 1 and the Approach 2 assessment have been performed, it is good practice to report the results of the Approach 2 analysis in addition to the results of Approach 1. Results of both Approach 1 and 2 should be used when setting priorities to inventory preparation. Figure 4.2, Decision Tree to identify key categories, illustrates how inventory compilers can determine which Approach to be used for the identification of key categories.

Figure 4.2 Decision Tree to identify key categories

Figure 4.2 Decision Tree to identify key categories

Box 4: Qualitative criteria

Any country that has developed a greenhouse gas inventory can perform Approach 1 Level Assessment to identify the categories whose level has a significant effect on total national emissions and removals. Those inventory compilers that have developed inventories for more than one year will also be able to perform Approach 1 Trend Assessment and identify categories that are key because of their contribution to the total trend of national emissions and removals.

4.3.1 Approach 1 to identify key categories

Approach 1 to identify key categories assesses the influence of various categories of sources and sinks on the level, and possibly the trend, of the national greenhouse gas inventory. When the inventory estimates are available for several years, it is good practice to assess the contribution of each category to both the level and trend of the national inventory. If only a single year's inventory is available, a level assessment should be performed.

Approach 1 can readily be accomplished using a spreadsheet analysis. Tables 4.2 and 4.3 in the following sections illustrate the format of the analysis. Separate spreadsheets are suggested for the level and trend assessments because it is necessary to sort the results of the analysis according to two different columns. It is more difficult to track the process if the analyses are combined in the same table. In both tables, columns A through D are inputs of the national inventory data. Section 4.5 illustrates the application of the Approach 1 to the Finnish inventory.

LEVEL ASSESSMENT

The contribution of each source or sink category to the total national inventory level is calculated according to Equation 4.1:

Equation 4.1 Level Assessment (Approach 1)

Key category level assessment = I source or sink category estimate I / total contribution

Where:

Lx,t

y y.t level assessment for source or sink x in latest inventory year (year t).

= absolute value of emission or removal estimate of source or sink category x in year t

= total contribution, which is the sum of the absolute values of emissions and removals in year t calculated using the aggregation level chosen by the country for key category analysis. Because both emissions and removals are entered with positive sign5, the total contribution/level can be larger than a country's total emissions less removals.6

Key categories according to Equation 4.1 are those that, when summed together in descending order of magnitude, add up to 95 percent of the sum of all Lxt.

Table 4.2 presents a spreadsheet that can be used for the level assessment. An example of the use of the spreadsheet is given in Section 4.5.

Table 4.2

Spreadsheet for the Approach 1 analysis - Level Assessment

A

B

C

D

E

F

Category

Greenhouse Gas

Latest Year Estimate

Ex,t

[in CO2-equivalent units]

Absolute Value of Latest Year Estimate

Lx,t

Cumulative Total of Column F

Total

y

1

Where:

Column A: code of IPCC categories (See Table 8.2 in Chapter 8, Reporting Guidance and Tables.) Column B : description of IPCC categories (See Table 8.2 in Chapter 8.) Column C : greenhouse gas from the category

5 Removals are entered as absolute values to avoid an oscillating cumulative value Lx,t as could be the case if removals were entered with negative signs, and thus to facilitate straightforward interpretation of the quantitative analysis.

This equation can be used in any situation, regardless of whether the national greenhouse gas inventory is a net source (as is most common) or a net sink.

Column D : value of emission or removal estimate of category x in latest inventory year (year t) in CO2-equivalent units

Column E : absolute value of emission or removal estimate of category x in year t

Column F : level assessment following Equation 4.1

Column G: cumulative total of Column F

Inputs to Columns A-D will be available from the inventory. The total of Column D presents the net emissions and removals. In Column E, absolute values are taken from each value in Column D. The sum of all entries in Column E is entered in the total line of Column E (note that this total may not be the same as the total net emissions and removals). In Column F, the level assessment is computed according to Equation 4.1. Once the entries in Column F are computed, the categories in the table should be sorted in descending order of magnitude according to Column F. After this step, the cumulative total summed in Column F can be calculated into Column G. Key categories are those that, when summed together in descending order of magnitude, add up to 95 percent of the total in Column G. Where the method is applied correctly, the sum of entries in Column F must be 1. The rationale for the choice of the 95 percent threshold for the Approach 1 builds on Rypdal and Flugsrud (2001) and is also presented in GPG2000, Section 7.2.1.1 in Chapter 7.

It is also good practice to examine categories identified between threshold of 95 percent and 97 percent carefully with respect to the qualitative criteria (see Section 4.3.3).

The level assessment should be performed for the base year of the inventory and for the latest inventory year (year t). If estimates for the base year have changed or been recalculated, the base year analysis should be updated. Key category analysis can also be updated for other recalculated years. In many cases, however, it is sufficient to derive conclusions regarding methodological choice, resource prioritisation or QA/QC procedures without an updated key category analysis for the entire inventory time series. Any category that meets the threshold for the base year or the most recent year should be identified as key. However, the interpretation of the results of the key category analysis should take longer time series than the most recent year into account if key category analyses are available. Because some categories having emissions/removals that fluctuate from year to year may be identified as key categories in one year but not in the next year. Therefore, for categories between thresholds of 95 and 97 percent it is suggested to compare the most recent key category analysis with the assessments for three or more previous years. If a category has been key for all or most previous years according to the either level or trend assessments or both (the two assessments should be considered separately), they should be identified as key in the latest year estimate except in cases where a clear explanation can be provided why a category may no longer be key in any future years. These additional categories should be addressed in the reporting table for key categories by using a column for comments (see Table 4.4 and reporting table for key categories in Section 4.4 for more information). The qualitative criteria presented in Section 4.3.3 may also help to identify which categories with fluctuating emissions or removals should be considered as key categories.

TREND ASSESSMENT

The purpose of the trend assessment is to identify categories that may not be large enough to be identified by the level assessment, but whose trend is significantly different from the trend of the overall inventory, and should therefore receive particular attention. The Trend Assessment can be calculated according to Equation 4.2 if more than one year of inventory data are available.

Equation 4.2 Trend Assessment (Approach 1)

Where:

Txt = trend assessment of source or sink category x in year t as compared to the base year (year 0) IeJ = absolute value of emission or removal estimate of source or sink category x in year 0 Ext and Ex0 = real values of estimates of source or sink category x in years t and 0, respectively £ Ey t and £ E 0 = total inventory estimates in years t and 0, respectively y y y

The trend of category refers to the change in the source or sink category emissions or removals over time, computed by subtracting the base year (year 0) estimate for source or sink category x from the latest inventory year (year t) estimate and dividing by the absolute value of the base year estimate.

The total trend refers to the change in the total inventory emissions (or removals) over time, computed by subtracting the base year (year 0) estimate for the total inventory from the latest year (year t) estimate and dividing by the absolute value of the base year estimate.

In circumstances where the base year emissions for a given category are zero, the expression may be reformulated to avoid zero in the denominator (see Equation 4.3).

Equation 4.3

Trend Assessment with zero base year emissions

x,t

Ex,t 1 z| Ey,0 1

y

The trend assessment identifies categories whose trend is different from the trend of the total inventory, regardless whether category trend is increasing or decreasing, or is a sink or source. Categories whose trend diverges most from the total trend should be identified as key, when this difference is weighted by the level of emissions or removals of the category in the base year.

Table 4.3 outlines a spreadsheet that can be used for the Approach 1 Trend Assessment.

Table 4.3

Spreadsheet for the Approach 1 analysis - Trend Assessment

A

B

C

D

E

F

G

Category

Greenhouse Gas

Base Year Estimate

Tx,t

Contribution to Trend

Cumulative

Total of Column G

Total

2 Ty,t y

1

Where:

Column A: code of IPCC categories (See Table 8.2 in Chapter 8.)

Column B : description of IPCC categories (See Table 8.2 in Chapter 8.)

Column C : greenhouse gas from the category

Column D: base year estimate of emissions or removals from the national inventory data, in CO2-equivalent units. Sources and sinks are entered as real values (positive or negative values, respectively).

Column E : latest year estimate of emissions or removals from the most recent national inventory data, in CO2-equivalent units. Sources and sinks are entered as real values (positive or negative values, respectively).

Column F : trend assessment from Equation 4.2 (from Equation 4.3 for zero base year emissions)

Column G: percentage contribution of the category to the total of trend assessments in last row of Column F, i.e., Tx,t / E Ty,t .

Column H: cumulative total of Column G, calculated after sorting the entries in descending order of magnitude according to Column G.

The entries in Columns A, B, C and E should be identical to those used in the Table 4.2, Spreadsheet for the Approach 1 analysis - Level Assessment. The base year estimate in Column D is always entered in the spreadsheet, while the latest year estimate in Column E will depend on the year of analysis. The value of Txt

(which is always positive) should be entered in Column F for each category of sources and sinks, following Equation 4.2, and the sum of all the entries entered in the total line of the table. The percentage contribution of each category to the total of Column F should be computed and entered in Column G. The categories (i.e., the rows of the table) should be sorted in descending order of magnitude, based on Column G. The cumulative total of Column G should then be computed in Column H. Key categories are those that, when summed together in descending order of magnitude, add up to more than 95 percent of the total of Column F. An example of Approach 1 analysis for the level and trend is given in Section 4.5.

The trend assessment treats increasing and decreasing trends similarly. However, for the prioritisation of resources, there may be specific circumstances where countries may not want to invest additional resources in the estimation of key categories with decreasing trends. Underlying reasons why a category showing strong decreasing trend could be key include activity decrease, mitigation measures leading to reduced emission factors or abatement measures (e.g., F-gases, chemical production) changing the production processes. In particular for a long-term decline of activities (not volatile economic trends) and when the category is not key from the level assessment, it is not always necessary to implement higher tier methods or to collect additional country-specific data if appropriate explanations can be provided why a category may not become more relevant again in the future. This could be the case e.g., for emissions from coal mining in some countries where considerable number of mines are closed or where certain production facilities are shut down. Regardless of the method chosen, countries should endeavour to use the same method for all years in a time series, and therefore it may be more appropriate to continue using a higher tier method if it had been used for previous years.

For other reasons of declining trends such as the introduction of abatement measures or other emission reduction measures, it is important to prioritise resources for the estimation of such categories that were identified as key in the trend assessment. Irrespective of the methodological choice, inventory compilers should clearly and precisely explain and document categories with strongly decreasing trends and should apply appropriate QA/QC procedures.

KEY CATEGORY ANALYSIS FOR A SUBSET OF INVENTORY ESTIMATES

The IPCC Good Practice Guidance for Land Use, Land-Use Change and Forestry (GPG-LULUCF, IPCC, 2003) provided guidance on how to conduct a key category analysis using a stepwise approach, identifying first the key (source) categories for the inventory excluding LULUCF (Land Use, Land-Use Change and Forestry), and secondly repeating the key category analysis for the full inventory including the LULUCF categories to identify additional key categories. This two step approach is now integrated into one general approach. However, inventory compilers may still want to conduct a key category analysis using a subset of inventory estimates. For example inventory compilers may choose to include only emission sources in order to exclude the effects of removals from the level assessment or in order to exclude the influence of different trends for carbon fluxes from the other emission trends (see examples in Tables 4.7 and 4.8). It is good practice to document on what subsets the analysis was performed and the differences in results comparing with an integrated analysis.

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