Guidance on uncertainty assessment in greenhouse gas inventories

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Guidance on uncertainty assessment in greenhouse gas inventories

Contents

1. Why assess uncertainty in a greenhouse gas inventory?
The Circular provides the following information on uncertainty:
1. Uncertainty in facility-level greenhouse gas inventories
2. Uncertainty in greenhouse gas inventories
3. Reducing uncertainty
2. Applying uncertainty calculations to greenhouse gas inventory results

1. Why assess uncertainty in a greenhouse gas inventory?

The quality of a greenhouse gas inventory depends on the accuracy of its calculated results. These results are affected by uncertainty in the activity data and emission factors used. The IPCC requires uncertainty in calculated results to be assessed and disclosed to interested parties. If uncertainty in calculated greenhouse gas results is high, data sources must be checked or data collection methods changed to increase confidence in the results and improve the reliability and accuracy of greenhouse gas emission quantification.

In Vietnam, Circular No. 17/2022/TT-BTNMT, issued by the Minister of Natural Resources and Environment on 15 November 2022 and effective from 15 February 2023, sets out technical regulations on measurement, reporting and verification of greenhouse gas emission reductions and greenhouse gas inventories in the waste management sector. It addresses uncertainty and requirements for calculating uncertainty when reporting quantified greenhouse gas emissions.

Download the Circular using the following link: Circular No. 17/2022/TT-BTNMT

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The Circular provides the following information on uncertainty:

Appendix II.3. Uncertainty assessment for facility-level greenhouse gas inventories

1. Uncertainty in facility-level greenhouse gas inventories

1.1. Definition

Uncertainty in a facility-level greenhouse gas inventory in the waste sector is a general, imprecise term referring to a lack of certainty in emissions-related data resulting from any causal factor, such as the use of unrepresentative factors, incomplete data methods for sources and sinks, or a lack of transparency.

1.2. Significance

Reported uncertainty information usually indicates quantitative estimates of possible or perceived differences involving the reported value, together with a qualitative description of the possible causes of those differences.

By their nature, inventory compilation, assessment and data collection involve uncertainties. Assessing these uncertainties is therefore necessary in greenhouse gas emissions reporting. This is not intended to question the validity of inventory data, but to establish the level of confidence. It also helps identify areas where reporting accuracy can be improved and informs methodological choices.

Total emissions reported by entities are often presented as a single number with an implicit or explicit confidence interval.

For example, reported total emissions of 125,000 tonnes of CO2 equivalent may be expressed more precisely as “total emissions are likely to be between 115,000 and 135,000 tonnes” or “total emissions are 125,000 tonnes, plus or minus 10%”. Uncertainty levels vary considerably between emission estimates, depending on the type of emission source, the calculation method used and the effort devoted to collecting and validating data.

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2. Uncertainty in greenhouse gas inventories

2.1. Causes of uncertainty

Even when the best available calculation methods are used, total greenhouse gas emissions remain subject to many sources of uncertainty:

Estimates used to compensate for missing data, such as facilities that do not report or missing fuel invoices;
Inaccurate measurement of activities that generate emissions;
Calculation errors and omissions;
Use of “average-case” emission factors that do not fully fit particular circumstances;
Assumptions used to simplify emission estimates for complex processes;
Use of approximate emission factors.

Greenhouse gas emission monitoring involves significant uncertainty because of several factors:

Some important waste treatment methods rely on complex processes, particularly biological processes, for which achieving the same accuracy as in other industrial sectors is difficult;
Some emissions are diffuse and are therefore estimated using theoretical mathematical models;
Treated waste has highly heterogeneous compositions, making statistical methods necessary and introducing significant but unavoidable variations. Default factors may be used, although their accuracy is unknown.

2.2. Default uncertainty values

The table below brings together uncertainty ranges associated with measuring equipment commonly used at waste management sites. It was developed using data supplied by experts from Veolia Environmental Services, Séché and Suez. The table is indicative and should only be used as default data when more precise information is unavailable from manufacturers or sites.

Table 1. Accuracy of measuring equipment

Equipment or measurement type	Example of use	Uncertainty	Observations
Flow measurement	Measuring the flow of natural gas used in an incinerator	2%	+ Commercial metering or equipment included in a preventive maintenance approach. + Manufacturer values cannot simply be used; actual operating and maintenance conditions must be considered. Calibration certificates and monitoring and maintenance documents should be retained.
	Measuring captured landfill gas	5-10%	+ Non-commercial measuring equipment used to monitor daily operations. + Corrective maintenance only.
	Measuring incinerator exhaust gas flow	5-10%	Difficult operating conditions, including meter location and variation in the measured flow; risks of equipment failure.
Weighbridge	Determining the weight of waste collected, treated or recycled	2%	+ Determining the weight of waste collected, treated or recycled + Commercial equipment or equipment included in a preventive maintenance programme. Calibration certificates and maintenance monitoring documents should be retained.
Fuel tanks	Visual indication of supplementary liquid fuel tank levels	10%	Uncertainty caused by imprecise methods of determining fuel oil levels or oil in water.
Analysers	Determining CO₂ content in exhaust gas using on-site equipment	5-10%	Difficult operating conditions, including equipment location; frequent risks of failure. However, the analysers are subject to strict regulatory monitoring.
	Determining fuel carbon content using a laboratory analyser (gas chromatography)	5%	+ Equipment requires preventive maintenance and periodic calibration. + Maintenance monitoring documents should be retained. + Sampling frequency must ensure that measured values are representative, and the choice of frequency must be documented.

It should also be noted that uncertainty principles apply to data obtained from measurements or analyses. This is why these principles cannot be applied to the modelling used to estimate methane emissions from landfills.

3. Reducing uncertainty

Uncertainty is inherent in preparing a greenhouse gas emissions inventory. Nevertheless, an entity must seek to reduce this uncertainty and keep the remaining uncertainty as low as possible. To do so, it must apply the following principles:

Ensure that measuring and analytical equipment, together with all resources needed to prepare the inventory, are appropriate and commonly used in the sector;
Perform preventive maintenance on measuring and analytical equipment, supported by procedures and records, to prevent potential measurement drift.

(source: Appendix II.3 Circular No. 17/2022/TT-BTNMT)