Uncertainties about emissions to air
Uncertainty in the emissions of greenhouse gases
The table below shows, for greenhouse gases, the uncertainties in the emissions for 2000 and in the emissions trend. The uncertainty for the listed emissions is expressed as the '95% confidence interval'. This means that the exact emission is not known but that it is very probably located within an interval consisting of the reported value plus or minus the stated percentage. The expression "very probably" is a standard from the IPCC for describing uncertainties. It indicates a statistical significance of 95%. For instance: the emission of carbon dioxide (CO2) of ? 3% means that it is not known exactly how much CO2 is emitted, but that the emission level is very probably in a range of 180 billion kg CO2 plus or minus 3%.
Uncertainties in the emissions of greenhouse gases
| Substance | 95% confidence interval | |
| Uncertainty in annual emissions1),3) | Trend uncertainty 1990-20002),3) | |
| % | ||
| CO2 | 3 | 3 |
| CH4 | 25 | 7 |
| N2O | 50 | 12 |
| F gases | 50 | 11 |
| CO2 equivalents | 5 | 34) |
| Source: Olivier et al., 2002 | RIVM/ EDC/Oct02 | |
| 1) Initial estimates of the uncertainties, corrected for potential interdependence of emission sources (Olivier et al., 2002). The uncertainties listed do not apply to emissions in 2001, which are provisional. 2) Analysis based on the trend data as reported in the Environmental Balance 2001. The trend for the F gases relates to the period 1995-2000. 3) For more information on the uncertainties in the annual emissions and in the emission trend see Olivier et al., 2002. 4) In this case a 3% uncertainty in the CO2 emission trend means that the range for the emission trend is 0-6%. | ||
Uncertainty in the emissions of acidifying substances
The 1999 values apply to the uncertainties in the emissions of acidifying substances into air. They are: 17% for ammonia (NH3), 11% for nitrogen oxides (NOx), 8% for sulphur dioxide (SO2) and 26% for volatile organic compounds (VOC). The uncertainty for the total amount of acid equivalents is approximately 9%. The estimate of the uncertainties for acidifying substances has not been adjusted for possible interdependence between emission sources. A study is currently being conducted into the uncertainties in the emission statistics for 2000 that will include this interdependence.
This has not yet been done for the emission into air of other substances, for example for the themes Eutrophication and Toxic and hazardous substances.
The uncertainties are determined using the Tier 1 methodology
The uncertainties in the annual emissions into ambient air (Environmental themes: Enhanced greenhouse effect and Acidification and transboundary air pollution) are determined using the Tier 1 methodology (IPCC, 2000). This is a simple standard method advised by the IPCC.
In the Tier 1 method, the uncertainties for the key sources are supplied by experts. Default values (IPCC, 2000) are used for uncertainties for smaller sources. This is therefore a top-down approach that is used because of the impossibility of obtaining uncertainty data using a bottom-up approach. It is assumed here that all distributions are normal, that there is no interdependence between sources and that the uncertainties are below 60%. Once the main causes of uncertainty have been identified, a more detailed method can be used for further examination: the IPCC Tier 2 method.
Originally, the methods were intended for reports about greenhouse gases from the IPCC. The methodology is now also used by the UNECE for transboundary air pollution (CLRTAP).
