View all publications

Annex 7: Uncertainty analysis (Table 6.1 of the IPCC Good Practice Guidance)

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 revised 1996 IPCC guidelines (IPCC, 1996) and good practice guidance (IPCC, 2000 and 2003) 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 Climate Change Convention inventory guidelines (UNFCCC, 2006) and IPCC good practice guidance (IPCC, 2000 and 2003). 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. 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 the emission or the removal factor uncertainty and zero in the activity data uncertainty

  • if uncertainty is not correlated across years, the uncertainty is entered as the uncertainty in the activity data and as zero in the emission or the removal factor uncertainty.

In the Tier 1 method, uncertainties in the trend are estimated using two sensitivities:

  • Type A sensitivity is the change in the difference of total emissions between the base year and the current year, expressed as a percentage. Further, this change results from a 1 per cent increase in emissions of a given source category and a greenhouse gas in both the base year and the current year.

  • Type B sensitivity is the change in the difference of total emissions between the base year and the current year, expressed as a percentage. Further, this change results 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 are associated with Type A sensitivities, and uncertainties that are not correlated between years are 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 in emissions since the base year. This is expressed as the number of percentage points in the 95 per cent confidence interval in the per cent change in emissions since the base year ie, “since 1990, gross emissions have gone up by 23 per cent ±5.5 per cent”. The total uncertainty in the trend is calculated by combining the contribution of emissions factor uncertainty and activity data uncertainty to the trend across all categories using equation 3.1 (IPCC, 2006).

The values for individual categories are an estimate of the uncertainty introduced into the trend by the category in question.

Table A7.1 The uncertainty calculation (including LULUCF removals) for New Zealand’s Greenhouse Gas Inventory 1990–2007 (IPCC Tier 1)

IPCC source category Gas Base year emissions or absolute value of removals  Year t emissions or absolute value of removals Activity data uncertainty Emission or removal factor uncertainty Combined uncertainty Combined uncertainty as a per cent of the national total in year t  Type A sensitivity Type B sensitvity Uncertainty in the trend in national total introduced by emission or removal factor uncertainty Uncertainty in trend in national total introduced by activity data uncertainty Uncertainty introduced into the trend in the national total
Energy sector CO2 22,593.0 31,559.9 5 0 5.0 1.6 0.0426 0.3861 0.0000 2.7301 2.7
Industrial processes sector  CO2 2,731.6 3,670.6 5 0 5.0 0.2 0.0034 0.0449 0.0000 0.3175 0.3
LULUCF sector - forest land CO2 18,673.8 24,565.2 5 25 25.5 6.2 0.0167 0.3005 0.4171 2.1250 2.2
LULUCF sector  other land use categories CO2 1,160.3 1,353.7 15 184 184.6 2.5 -0.0011 0.0166 -0.1976 0.3513 0.4
CRF6C - waste incineration CO2 12.9 0.9 10 40 41.2 0.0 -0.0002 0.0000 -0.0074 0.0002 0.0
Energy sector  CH4 716.4 831.9 5 50 50.2 0.4 -0.0007 0.0102 -0.0355 0.0720 0.1
CRF2A - mineral products CH4 0.0 0.0                  
CRF2B - chemical industry CH4 20.2 18.3 0 80 80.0 0.0 -0.0001 0.0002 -0.0066 0.0000 0.0
CRF4A - enteric fermentation CH4 21,819.0 23,326.4 2 53 53.0 12.2 -0.0461 0.2854 -2.4438 0.8071 2.6
CRF4B - manure management CH4 578.7 729.1 2 100 100.0 0.7 0.0001 0.0089 0.0125 0.0252 0.0
CRF4E- prescribed burning CH4 2.7 0.9 20 60 63.2 0.0 0.0000 0.0000 -0.0019 0.0003 0.0
CRF4F - burning of residues CH4 21.4 13.2 50 40 64.0 0.0 -0.0002 0.0002 -0.0066 0.0114 0.0
LULUCF sector CH4 49.9 62.1 10 35 36.4 0.0 0.0000 0.0008 0.0001 0.0107 0.0
CRF 6A - solid waste disposal CH4 2,063.2 1,438.0 0 20 20.0 0.3 -0.0138 0.0176 -0.2752 0.0000 0.3
CRF 6B - wastewater handling CH4 207.0 202.5 0 20 20.0 0.0 -0.0007 0.0025 -0.0134 0.0000 0.0
CRF6C - waste incineration CH4 0.0 0.0 10 100 100.5 0.0 0.0000 0.0000 0.0000 0.0000 0.0
Energy sector  N2O 143.4 261.3 5 50 50.2 0.1 0.0010 0.0032 0.0509 0.0226 0.1
Solvents - N2O use N2O 41.5 43.4 10 0 10.0 0.0 -0.0001 0.0005 0.0000 0.0075 0.0
CRF4D - agricultural soils N2O 10,043.4 12,298.1 5 73 73.2 8.9 -0.0022 0.1504 -0.1595 1.0638 1.1
CRF4B - manure management N2O 38.0 58.0 5 100 100.1 0.1 0.0001 0.0007 0.0131 0.0050 0.0
CRF4E - prescribed burning N2O 0.5 0.2 20 60 63.2 0.0 0.0000 0.0000 -0.0003 0.0001 0.0
CRF4F - burning of residues N2O 7.4 4.3 50 40 64.0 0.0 -0.0001 0.0001 -0.0024 0.0037 0.0
LULUCF sector  N2O 5.7 17.0 10 35 36.4 0.0 0.0001 0.0002 0.0043 0.0029 0.0
CRF6B - wastewater handling N2O 153.5 179.2 0 1200 1200.0 2.1 -0.0001 0.0022 -0.1683 0.0000 0.2
CRF6C - waste incineration N2O 1.6 1.2 10 100 100.5 0.0 0.0000 0.0000 -0.0010 0.0002 0.0
CRF2F HFCs 0.0 856.6 24 0 24.0 0.2 0.0105 0.0105 0.0000 0.3557 0.4
CRF2C PFCs 642.2 41.7 0 30 30.0 0.0 -0.0092 0.0005 -0.2775 0.0000 0.3
CRF2F SF6 15.2 14.7 5 20 20.6 0.0 -0.0001 0.0002 -0.0010 0.0013 0.0
                         
Total emissions/removals   81,742.5 101,548.2   Uncertainty in the year 16.7%   Uncertainty in the trend 4.5%

assumptions as per IPCC GPG

  1. where only total uncertainty is known for a source category then the following rules have been used.
    1. if uncertainity is assumed to be correlated across years, then total uncertainty is entered as emission factor uncertainty 
    2. if uncertainty is assumed not to be correlated across years, then the total uncertainty is entered as activity data uncertainty 
  2. Column K: The same emission factor is used in both years and the emission factors are fully correlated.  
  3. Column L: The activity data in both years is assumed independent (not correlated).

Note that the total shown will not equal the gross or net totals reported in the CRF/NIR as not every source is specified in the uncertainty analysis

Table A7.2 The uncertainty calculation (excluding LULUCF removals) for New Zealand’s Greenhouse Gas Inventory 1990–2007 (IPCC Tier 1)

IPCC source category Gas Base year emissions   Year t emissions Activity data uncertainty Emission factor uncertainty Combined uncertainty Combined uncertainty as a % of the total emissions in year t  Type A sensitivity Type B sensitvity Uncertainty in the trend in national totals introduced by emission factor uncertainty Uncertainty in trend in national total introduced by activity data uncertainty Uncertainty introduced into the trend in total emissions
Energy sector CO2 22,593.0 31,559.9 5 0 5 2.1 0.0637 0.5098 0.0000 3.6047 3.6
Industrial processes sector  CO2 2,731.6 3,670.6 5 0 5 0.2 0.0054 0.0593 0.0000 0.4193 0.4
CRF6C - waste incineration CO2 12.9 0.9 10 40 41 0.0 -0.0002 0.0000 -0.0096 0.0002 0.0
Energy sector  CH4 716.4 831.9 5 50 50 0.6 -0.0007 0.0134 -0.0350 0.0950 0.1
CRF2A - mineral products CH4 0.0 0.0                  
CRF2B - chemical industry CH4 20.2 18.3 0 80 80 0.0 -0.0001 0.0003 -0.0082 0.0000 0.0
CRF4A - enteric fermentation CH4 21,819.0 23,326.4 2 53 53 16.4 -0.0536 0.3768 -2.8394 1.0657 3.0
CRF4B - manure management CH4 578.7 729.1 2 100 100 1.0 0.0004 0.0118 0.0358 0.0333 0.0
CRF4E - prescribed burning CH4 2.7 0.9 20 60 63 0.0 0.0000 0.0000 -0.0024 0.0004 0.0
CRF4F - burning of residues CH4 21.4 13.2 50 40 64 0.0 -0.0002 0.0002 -0.0084 0.0150 0.0
LULUCF sector CH4 49.9 62.1 10 35 36 0.0 0.0000 0.0010 0.0007 0.0142 0.0
CRF 6A - solid waste disposal CH4 2,063.2 1,438.0 0 20 20 0.4 -0.0175 0.0232 -0.3496 0.0000 0.3
CRF 6B - wastewater handling CH4 207.0 202.5 0 20 20 0.1 -0.0008 0.0033 -0.0163 0.0000 0.0
CRF6C - waste incineration CH4 0.0 0.0 10 100 100 0.0 0.0000 0.0000 0.0000 0.0000 0.0
Energy sector  N2O 143.4 261.3 5 50 50 0.2 0.0014 0.0042 0.0695 0.0298 0.1
Solvents - N2O use N2O 41.5 43.4 10 0 10 0.0 -0.0001 0.0007 0.0000 0.0099 0.0
CRF4D - agricultural soils N2O 10,043.4 12,298.1 5 73 73 11.9 0.0005 0.1986 0.0338 1.4047 1.4
CRF4B - manure management N2O 38.0 58.0 5 100 100 0.1 0.0002 0.0009 0.0185 0.0066 0.0
CRF4E- prescribed burning N2O 0.5 0.2 20 60 63 0.0 0.0000 0.0000 -0.0004 0.0001 0.0
CRF4F - burning of residues N2O 7.4 4.3 50 40 64 0.0 -0.0001 0.0001 -0.0030 0.0049 0.0
LULUCF sector  N2O 5.7 17.0 10 35 36 0.0 0.0002 0.0003 0.0057 0.0039 0.0
CRF6B - wastewater handling N2O 153.5 179.2 0 1200 1200 2.8 -0.0001 0.0029 -0.1607 0.0000 0.2
CRF6C - waste incineration N2O 1.6 1.2 10 100 100 0.0 0.0000 0.0000 -0.0012 0.0003 0.0
CRF2F HFCs 0.0 856.6 120 50 130 1.5 0.0138 0.0138 0.6919 2.3482 2.4
CRF2C PFCs 642.2 41.7 0 30 30 0.0 -0.0120 0.0007 -0.3600 0.0000 0.4
CRF2F SF6 15.2 14.7 5 20 21 0.0 -0.0001 0.0002 -0.0013 0.0017 0.0
                         
Total emissions/removals   61,908.4 75,629.3   Uncertainty in the year 20.6%   Uncertainty in the trend 5.5%

assumptions as per IPCC GPG

  1. where only total uncertainty is known for a source category then the following rules have been used.
    1. if uncertainity is assumed to be correlated across years, then total uncertainty is entered as emission factor uncertainty 
    2. if uncertainty is assumed not to be correlated across years, then the total uncertainty is entered as activity data uncertainty 
  2. Column K: The same emission factor is used in both years and the emission factors are fully correlated.  
  3. Column L: The activity data in both years is assumed independent (not correlated).

Note that the total emissions shown will not equal the total reported in the CRF/NIR as not every source is specified in the uncertainty analysis