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  • Chapter 6 Quantifying Uncertainties in Practice

    IPCC Good Practice Guidance and Uncertainty Management in National Greenhouse Gas Inventories 6.1

    6

    QUANTIFYING UNCERTAINTIES INPRACTICE

  • Quantifying Uncertainties in Practice Chapter 6

    IPCC Good Practice Guidance and Uncertainty Management in National Greenhouse Gas Inventories6.2

    CO-CHAIRS, EDITORS AND EXPERTS

    Co-chairs of the Expert Meeting on Cross-sectoral Methodologies forUncertainty Estimation and Inventory QualityTaka Hiraishi (Japan) and Buruhani Nyenzi (Tanzania)

    REVIEW EDITORRichard Odingo (Kenya)

    Expert Group on Quantifying Uncertainties in PracticeCO-CHAIRSJim Penman (UK) and Semere Habetsion (Eritrea)

    AUTHORS OF BACKGROUND PAPERSKay Abel (Australia), Simon Eggleston (UK) and Tinus Pullus (Netherlands)

    CONTRIBUTORSSimon Eggleston (UK), Christopher Frey (USA), Kari Gronfors (Finland), Niklas Hhne (UNFCCC Secretariat),Charles Jubb (Australia), Katarina Mareckova (Slovak Republic), Jeroen Meijer (IPCC-NGGIP/TSU), FrankNeitzert (Canada), Todd Ngara (Zimbabwe), Tinus Pulles (Netherlands), Emmanuel Rivire (France), ArthurRypinski (USA), Martiros Tsarukyan (Armenia), and Peter Zhou (Botswana)

  • Chapter 6 Quantifying Uncertainties in Practice

    IPCC Good Practice Guidance and Uncertainty Management in National Greenhouse Gas Inventories 6.3

    C o n t e n t s

    6 QUANTIFYING UNCERTAINTIES IN PRACTICE

    6.1 OVERVIEW..........................................................................................................................................6.5

    6.2 IDENTIFYING UNCERTAINTIES .....................................................................................................6.6

    6.2.1 Uncertainties associated with continuous monitoring of emissions ...........................................6.6

    6.2.2 Uncertainties associated with direct determination of emission factors.....................................6.7

    6.2.3 Uncertainties associated with emission factors from published references................................6.7

    6.2.4 Uncertainties associated with activity data ................................................................................6.8

    6.2.5 Expert Judgement ......................................................................................................................6.9

    6.3 METHODS FOR COMBINING UNCERTAINTIES.........................................................................6.12

    6.3.1 Comparison between tiers and choice of method.....................................................................6.13

    6.3.2 Tier 1 Estimating uncertainties by source category with simplifying assumptions ...............6.13

    6.3.3 Tier 1 aggregation and reporting .............................................................................................6.18

    6.4 TIER 2 ESTIMATING UNCERTAINTIES BY SOURCE CATEGORYUSING MONTE CARLO ANALYSIS...............................................................................................6.18

    6.4.1 Tier 2 uncertainties in trends ...................................................................................................6.21

    6.4.2 Reporting Tier 2 uncertainty analysis ......................................................................................6.22

    6.5 PRACTICAL CONSIDERATIONS IN USING MONTE CARLO SIMULATION ..........................6.24

    6.5.1 Specifying probability distributions for inventory inputs.........................................................6.24

    6.5.2 How much effort is needed to characterise uncertainty in an inventory input?........................6.25

    6.5.3 Choosing a simulation technique and a simulation sample size...............................................6.26

    6.5.4 Dependence and correlation among inventory inputs ..............................................................6.26

    6.5.5 Does correlation matter?..........................................................................................................6.26

    6.5.6 Some methods for dealing with dependencies or correlation...................................................6.27

    6.5.7 Specifying correlation in inventory inputs ...............................................................................6.27

    6.5.8 Analysing inventory outputs ....................................................................................................6.27

    6.5.9 Encouragement of use of appropriate techniques ....................................................................6.27

    6.6 CONCLUSION ...................................................................................................................................6.27

    APPENDIX 6A.1 DERIVATION OF THE FORMULAE IN TABLE 6.1 (TIER 1)...............................6.28

    APPENDIX 6A.2 TIER 1 UNCERTAINTY CALCULATION EXAMPLE ...........................................6.31

    REFERENCES ............................................................................................................................................6.34

  • Quantifying Uncertainties in Practice Chapter 6

    IPCC Good Practice Guidance and Uncertainty Management in National Greenhouse Gas Inventories6.4

    F i g u r e s

    Figure 6.1 Illustration of Monte Carlo Method ...............................................................................6.20

    Figure 6.2 Example Frequency Plots of the Results of a Monte Carlo Simulation .........................6.21

    Figure 6.3 Calculation Scheme for Monte Carlo Analysis of the Absolute Emissionsand the Trend of a Single Source Category, Estimated as Emission FactorTimes an Activity Rate ..................................................................................................6.22

    T a b l e s

    Table 6.1 Tier 1 Uncertainty Calculation and Reporting ...............................................................6.16

    Table 6.2 Tier 2 Uncertainty Reporting .........................................................................................6.23

    Table 6.3 Tier 1 Uncertainty Calculation and Reporting Example ................................................6.31

  • Chapter 6 Quantifying Uncertainties in Practice

    IPCC Good Practice Guidance and Uncertainty Management in National Greenhouse Gas Inventories 6.5

    6 QUANTIFYING UNCERTAINTIES INPRACTICE

    6 . 1 O V E R V I E WThis chapter describes good practice in estimating and reporting uncertainties associated with both annualestimates of emissions, and emission trends over time. It identifies types of uncertainty from the viewpoint of theinventory practitioner and shows how to obtain expert judgements in a consistent manner. It provides two tiersfor combining source category uncertainties into an uncertainty estimate for total national emissions, and presentsan example of the application of the Tier 1 method.

    The chapter is consistent with source-specific good practice guidance described in Chapters 2 to 5, the generalprinciples discussed in Annex 1, Conceptual Basis for Uncertainty Analysis, and the chapters on methodologicalchoice (Chapter 7, Methodological Choice and Recalculation) and QA/QC (Chapter 8, Quality Assurance andQuality Control).

    Uncertainty estimates are an essential element of a complete emissions inventory. Uncertainty information is notintended to dispute the validity of the inventory estimates, but to help prioritise efforts to improve the accuracy ofinventories in the future and guide decisions on methodological choice, as indicated in Chapter 7,Methodological Choice and Recalculation. Inventory practitioners understand that for most countries and sourcecategories, greenhouse gas emissions estimates are reasonably accurate. However, national inventories preparedpursuant to the Revised 1996 IPCC Guidelines for National Greenhouse Gas Inventories (IPCC Guidelines) willtypically contain a wide range of emission estimates, varying from carefully measured and demonstrablycomplete data on emissions of certain engineered chemicals, to order-of-magnitude estimates of highly variablenitrous oxide (N2O) fluxes from soils and waterways.

    Inventory estimates can be used for a range of purposes. For some purposes, only the national total matters, whilefor others, the detail by greenhouse gas and source category is important. In order to match the data to theintended purpose, users must be able to understand the actual reliability of both the total estimate and itscomponent parts. For this reason, the methods used to communicate uncertainty must be practical, scientificallydefensible, robust enough to be applicable to a range of source categories, methods and national circumstances,and presented in ways comprehensible to non-specialist inventory users.

    There are many reasons that actual emissions and sinks may differ from the number calculated in a nationalinventory. These reasons are discussed at greater length in Annex 1. Some sources of uncertainty (i.e. samplingerror or limitations on instrument accuracy) may generate well-defined, easily characterised estimates of therange of potential error. However, other sources of uncertainty may be much more difficult to characterise. Thischapter describes how to account for both well-defined statistical uncertainties and less specific informationcharacterising other forms of uncertainty, and how to combine this information into a characterisation of theuncertainty of both the total inventory and its components.

    Ideally, emissions estimates and uncertainty ranges would both be derived from source-specific measured data.Since it is not practical to measure every emission source in this way, estimates are often based on the knowncharacteristi