Uncertainty estimates are an essential element of a complete emissions inventory. The purpose of uncertainty information is not to dispute the validity of the inventory estimates, but to help prioritise efforts to improve the accuracy of inventories in the future and guide decisions on methodological choice (IPCC, 2000). Good Practice Guidance also notes that inventories prepared following the IPCC Guidelines and Good Practice Guidance will typically contain a wide range of emission estimates, varying from carefully measured and demonstrably complete data on emissions to order-of-magnitude estimates of highly variable N2O fluxes from soils and waterways (IPCC, 2000).
New Zealand has included a Tier 1 uncertainty analysis as required by the inventory guidelines (FCCC/SBSTA/2004/8) and Good Practice Guidance (IPCC, 2000). Uncertainties in the categories are combined to provide uncertainty estimates for the entire inventory in any year and the uncertainty in the overall inventory trend over time. The New Zealand methodology follows the Tier 1 calculation procedure as specified in Good Practice Guidance (IPCC, 2000). LULUCF categories have been included using the absolute value of any removals of CO2 (table A7.1). Table A7.2 calculates the uncertainty only in emissions, ie, excluding LULUCF removals.
A7.1 Tier 1 uncertainty calculation
The uncertainty in activity data and emission/removal factors shown in table A7.1 and A7.2 are equal to half the 95 per cent confidence interval divided by the mean and expressed as a percentage. The reason for halving the 95 per cent confidence interval is that the value corresponds to the familiar plus or minus value when uncertainties are loosely quoted as “plus or minus x per cent”. Where uncertainty is highly asymmetrical, the larger percentage difference between the mean and the confidence limit is entered. Where only the total uncertainty is known for a category then:
if uncertainty is correlated across years, the uncertainty is entered as emission or removal factor uncertainty and 0 in activity data uncertainty
if uncertainty is not correlated across years, the uncertainty is entered as uncertainty in activity data and 0 in emission or removal factor uncertainty.
In tables A7.1 and A7.2, the figure labelled “uncertainty in the year” is an estimate of the percentage uncertainty in total national emissions and removals in the current year. This figure is calculated from the entries for individual categories combined by summing the squares of all the entries in the column “combined uncertainty as a % of the national total in year t” and taking the square root.
In the Tier 1 methodology, uncertainties in the trend are estimated using two sensitivities:
Type A sensitivity: the change in the difference in the national total between the base year and the current year, expressed as a percentage, resulting from a 1 per cent increase in emissions of a given source category and gas in both the base year and the current year.
Type B sensitivity: the change in the difference in overall emissions between the base year and the current year, expressed as a percentage, resulting from a 1 per cent increase in emissions of a given source category and gas in the current year only.
Uncertainties that are fully correlated between years will be associated with Type A sensitivities, and uncertainties that are not correlated between years will be associated with Type B sensitivities.
In tables A7.1 and A7.2, the figure labelled “uncertainty in the trend” is an estimate of the total uncertainty in the trend, calculated from the entries above by summing the squares of all the entries and taking the square root. The values for the individual categories are an estimate of the uncertainty introduced into the trend by the category in question.
Table A7.1 Uncertainty calculation for the New Zealand Greenhouse Gas Inventory 1990–2005 including LULUCF removals (following IPCC Tier 1)
Table A7.2 Uncertainty calculation for the New Zealand Greenhouse Gas Inventory 1990–2005 excluding LULUCF removals (following IPCC Tier 1)