Accuracy of GHG emission estimates

Due to the complex nature of estimating emissions inventories, no simple quantitative treatment can be given for the accuracy of emission estimates. However, a qualitative indication can be made of the systematic errors. Some of the more important factors resulting in uncertainties in greenhouse gas emission estimates are:

• Lack of measurements of emission factors for UK plant and conditions;
• Limited samples of plant and vehicles where UK measurements have been made;
• Short period measurements being used to estimate annual averages;
• Uncertainties in fuel consumption data, for example data may be deliveries rather than consumption;
• Uncertainties in secondary data which are used to estimate emissions. For example, vehicle-kilometres used in calculating road transport emissions are often derived from sample surveys which are themselves subject to uncertainties.

For any given year considerable uncertainties surround the emissions estimates for each pollutant, however trends over time are likely to be more reliable. Table 2.10 gives some assessment of the uncertainty of the inventories. These data are given taken from a more thorough examination in Eggleston et al (1998).

Table 2.10 Uncertainty of the Emission Inventories

a - Skewed uncertainty distribution.

It should be noted that these uncertainties primarily arise from emission factor uncertainties. Activity data is considered to be more reliable and better characterised. As a result it can be assumed that the trends identified from time series plots are considerably more reliable than an absolute emission total.