Treatment of Uncertainties for National Estimates of Greenhouse Gas Emissions
A2 Uncertainty Evaluation: Other Specific National Approaches
A number of approaches adopted in other countries to evaluate uncertainties in emissions estimates were reviewed briefly.
Australia
It is reported [72] that various levels of uncertainty in emission estimates are associated with different sections of the inventory as a whole, although no information is given on the methodology used to estimate such uncertainties. It is stated that difficulty in obtaining accurate statistics and relevant data greatly hinders the process of estimation, be it in qualitative or quantitative form. Notwithstanding, it is recognised that the emission estimates for transport are likely to be subject to a relatively small uncertainty (~10%), whereas others, such as the emissions due to land use change and forestry, are subject to a much larger uncertainty (about a factor of two). Research is underway to refine estimates of emissions in areas believed to be subject to the greatest uncertainties.
Canada
Uncertainties in emission estimates of greenhouse gases are a major concern to the Canadian authorities [73]. Although it is recognised that there are many causes of uncertainties, most are believed to be due to the following:
- differences in interpretation of source and sink category definitions, assumptions, units, etc.
- inadequate and incorrect socio-economic activity data used
- inappropriate application of emission factors
- empirical uncertainty 33 of measured emission data and basic processes leading to emissions
The document refers to the use of mean and standard deviations of expert estimates of sectoral emissions. A document summarising the full methodology for assessing uncertainties [61] has been requested for use in this work. The overall uncertainties for the three main greenhouse gases were developed using a stochastic (probabilistic) model, and are estimated to be about 4% for CO2, 30% for CH4 and 40% for N2O. Uncertainties in individual sectors are acknowledged to be much higher than these figures. Overall uncertainties in the emissions of CO2, which dominate, are believed to be relatively low.
Denmark
The approach adopted by Denmark in assessing uncertainties in greenhouse gas inventories is not given in any meaningful detail [74]. Uncertainties in emission estimates are recognised as arising from two sources: uncertainty in the statistics and uncertainty in the emission factors used. The emission factors are based either on calculations, as is the case for carbon dioxide (from energy and carbon content), or more directly as inferred from measurements. It is recognised that a further uncertainty arises from a lack of completeness in the available data.
The greatest uncertainties in estimates arise for the non-methane volatile organic compounds, methane, and nitrous oxide, with an uncertainty factor of about 2. With the carbon monoxide and oxides of nitrogen inventories, the uncertainty is assumed to be less than 30-40%. The carbon dioxide uncertainty may be as low as 1-2%. No information is given on how these numerical values were derived.
Germany
It is widely recognised [75] that emissions data are subject to considerable uncertainty, due to lack of information on emissions-causing processes, and (more importantly) a lack of data on the extent of certain emission-relevant activities. Much effort has been devoted in Germany to composing scenarios of such future activities.
Emission factors for non-CO2 sources have been based on measurements made under defined conditions, although the number of measurements is regarded as inadequate. This also applies to non-energy-related emissions. Emissions from combustion-related activities are considered to be much more reliable.
Beyond stating that research is being focused on improving the quality of the information used in composing national estimates, the information given in the above reference does not permit the reader to deduce in any way the method or methods used to assess the level of uncertainties in inventory estimates of greenhouse gases in Germany.
The Netherlands
Numerical uncertainty estimates for the inventories of greenhouse gases are given [76], although it is not stated how these were derived. A combination of expert opinion and some numerical manipulation is therefore suspected.
- Uncertainties in emission factors for CO
2 are believed to be of the order of 2%, with overall uncertainties in emissions will 'not be much larger than a few percent'.
Uncertainties in CH4 emissions from most sectors is estimated to be about 25%, with the exception for uncertainties from waste, estimated to be about 30%. The overall estimate is about 25%.
N2O emissions are subject to much larger uncertainties, and revisions are like in the near future. Emission factor uncertainties are of the order of 50 to 100%, and several sources are believed to be as yet unidentified. An overall uncertainty of about 50% is estimated.
In the case of partially halogenated hydrofluorocarbons (HFCs) and perfluorocarbons (PFCs), owing to the fact that these materials are being phased out the uncertainties are derived not only from emission factors and activity rates, but also the duration of storage and leakage rates. Activity rates here are well characterised; the uncertainties are determined rather by factors elsewhere in the overall assessment. The overall uncertainties of HFC and SF6 are of the order of 50%, whereas those for PFCs are higher (~100%).
In the recent document outlining the Netherlands' emissions data [77], the percentages given above are reproduced without comment, together with the more qualitative indicators of data quality promoted by the IPCC (see Section 2.1.2(b)).
Portugal
The official communication [78] produced in the context of the IPCC requirements recognises the issue of uncertainties in emission estimates. Reliability estimates are given in conformity with the format given [79], and 'average' (i.e. expected) actual degrees of reliability were recorded down to the sub-classification level 34 .
Sweden
The current emission inventory follows the IPCC guidelines for reporting to the UN Climate Convention [80], and in the context of uncertainties, is restricted to qualitative data quality indicators. Official Swedish emission statistics do not include data on emissions of N2O, CH4 and CO. Estimates of the size of the emissions are therefore highly uncertain in many cases.
2 emissions (which contribute about 80% to the total global warming potential for Sweden) are of relatively high reliability; the problem exists more in the topic of sequestration from land use changes (notably forest growth).
Agriculture and landfilling of wastes contribute over 90% of the emissions of CH4. Emissions are calculated using national emission factors. The uncertainties in the contribution from landfills (~20% of the total) are particularly marked. Overall data for CH4 is recorded as having medium reliability.
Together with fertiliser manufacture, the largest emissions of N2O come from a number of combustion processes. Emissions from agricultural land are estimates using IPCC emission factors. Overall data for N2O must regarded as having low reliability.
No comment is made on the uncertainties in emissions of HFCs, PFCs and SF6.
Emissions of NOx (mainly from combustion) are recorded as having a high degree of reliability.
The statistics of CO production, again resulting mainly from combustion, are much less comprehensive than the above, and warrant an overall 'medium' in terms of reliability.
Volatile organic compounds (VOCs) are released from a number of processes. Owing to the considerable difficulties that exist in characterising some of these, the data must be regarded on the whole as having low reliability.
United States of America
Key sources of uncertainty that affect the projection of future emissions are listed [81], but no attempt is made to quantify these projections further, or current uncertainties in estimates. The factors themselves comprise, Climate Change Action Program funding, legislation, energy prices, economic growth, electricity supply demand, forest carbon sequestration and current climate. Most of these factors are acknowledged to be able to affect emissions either positively or negatively.
A3 Conclusions of Review of Approaches
The methods reviewed above span the entire range and provenance of information in an emission inventory. By way of a summary, Table A8 presents the features and benefits of the main types of the analytical methods, and indicates the relevance of each in the present context. This review of approaches was completed prior to the completion of the full assessment of the uncertainties in the emissions inventory. As such, it was not clear which of the reviewed methods would prove to be most useful in the subsequent analysis. Where the quality of information is most satisfactory, it was the intention to use the @RISK probabilistic tool to investigate how the uncertainties in the input parameters are transmitted through to the final inventory. Where the data quality proved to be less satisfactory, the less quantitative methods would be considered, with the semi-quantitative techniques and rating systems considered as a priority.
In the event, as noted in the Introduction of the main report, sufficient information was uncovered, or sufficiently robust judgements could be made, to justify the use throughout this study of the quantitative methods outlined in Section A1.3 of this Appendix. As this proved possible, the potentially conceptual difficulty of how to present some overall measure of uncertainty when this is contributed to by components which have varying degrees of quantitativeness did not arise. The issue of the variability of parameter values about a mean, as opposed to uncertainty around a 'true' value, however, did remain.
