ANNEX 1
MANAGING UNCERTAINTIES
Uncertainties are inevitable in any estimate of national emissions or removals.
Some important causes of uncertainty are:•differing interpretations of source and sink category or other definitions,assumptions, units etc.

•

use of simplified representations with "averaged" values, especially emission
factors and related assumptions to represent characteristics of a given
population•

uncertainty in the basic socio-economic activity data which drives thecalculations•

inherent uncertainty in the scientific understanding of the basic processes
leading to emissions and removals.

A major objective of the IPCC methodology is to help national experts reduce
uncertainty in their greenhouse gas inventories to the minimum level possible.
However, the approach also recognises that significant uncertainties will remain
despite these efforts, and that these uncertainties will vary widely:•

between different greenhouse gases•

between source categories for each gas•

between countries reporting the same gases and sources (depending on
approach, levels of detail, use of default or country specific data etc.)

It is important to provide as thorough an understanding as possible of theuncertainties involved when estimates are provided for scientific or policy uses. A
simple method for expressing the confidence or uncertainty of point estimatesqualitatively is given elsewhere in the Reporting Instructions. However it is more
useful to express uncertainty quantitatively and systematically in the form ofwell developed confidence intervals. This Annex provides some initial suggestions
for developing quantitative uncertainty information. However, at present, it is
only possible to provide a conceptual framework which relies on users to supply
statistical data or equivalent expert judgement. IPCC/OECD consider theconsistent estimation of uncertainty to be critically important, and will make it
the focus of future work. Individual experts are encouraged to estimate
uncertainty ranges as well as possible and to report results with their inventories.
This will be of assistance with the ongoing work of developing methods.

A1.1 Sources of Uncertainty
Definitions
Use of the IPCC Reporting Instructions will minimise variability or uncertainty
which would otherwise be introduced by issues of definition. The IPCC
Reporting Instructions provides common definitions of source categories andRevised 1996 IPCC Guidelines for National Greenhouse Gas Inventories:

Reporting Instructions

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MANAGING UNCERTAINTIES

other terms, units, procedures, etc. The source categories are set out in
Chapter 1 Understanding the Common Reporting Framework.

Estimation Methodology
The IPCC/OECD programme has sought consensus among researchers,
sectoral interest groups and national technical experts on the bestpracticable default estimation procedures for priority gases and sources.
These default methodologies are described in Volume 2 of the Guidelines,
the Greenhouse Gas Inventory Workbook. By using these methods countries
can minimise variations or uncertainties in national estimates which wouldbe introduced by a choice of methodology. However, it must be recognisedthat default methods represent a compromise between the level of detailwhich would be needed to create the most accurate estimates for eachcountry and the input data likely to be available or readily obtainable inmost countries. In many cases, the simplest default methods aresimplifications with general default values which introduce large
uncertainties into a national estimate. Within many of the default methods
different optional levels of detail are provided to reflect whether users havedetailed data for their national situation or have to rely strictly on general
default values. There may be considerable variation in how well the general
default values represent conditions of the actual population of source
activities in a particular country. For example, the uncertainty relating todefault carbon emission coefficients for the global population of fossil fuelcombustion sources may be characterised as quite low (5-10 per cent) in
the IPCC methodology; but national experts for a particular country may
know that the characteristics of such fuels in their country vary widely fromglobal average values. In such a country, use of default values would
introduce a larger uncertainty. Thus, even for the simplest application of thedefault methods, it is not possible to provide general uncertainty values for
all countries.
The Reference Manual provides more options, including ways of doing
calculations at greater levels of detail and, in some cases, alternativemethodologies. Users of the IPCC Guidelines may use their own methodologies
if they believe these will provide more accurate results for their nationalsituation. Alternative methods should be carefully documented and results
reported in the standard IPCC source and sink categories. Documentation ofalternative methods may involve presentation of new empirical data which may
in turn provide a basis for the improvement of the default procedures and data.
However, whichever methods are used - default methods, more detailed
versions of default methods, or entirely different methods - users should
determine as far as possible the ranges of uncertainty introduced by theemission factors and other input assumptions used, whatever their source.

Socio-economic Activity Data
The IPCC default methodologies identify activity data from internationalsocio-economic data series wherever possible. International compilations ofsocio-economic activity data do not generally include quantitative
uncertainty estimates around country-level data summaries. Some of thenational sources that provide data to the international series may haveA1.2Revised 1996 IPCC Guidelines for National Greenhouse Gas Inventories:

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quantified uncertainty for their own national data. As with uncertainty inmethodology and emission factors, the inventory developers must judge thequality of activity data used in their own national inventory.

Underlying Scientific Understanding
Current scientific understanding of the various human-induced processes
which lead to emissions and removals of greenhouse gases to and from theatmosphere is incomplete. In some cases, where substantial measurement
data exist and have been thoroughly analysed, this understanding provides abasis for accurate calculations of global and national emissions. In manycases, however, data and analysis have not attained this state. This variation
affects the uncertainty inherent in the various components of the default
methods, as well as the estimates using other methodologies. Table A1-1
provides an illustrative assessment of the relative uncertainties in thescientific basis for global emission estimates for some key components of
the IPCC methodology. The overall uncertainty ranges shown here arebased on an interpretation of the uncertainty information presented by theIPCC (1992). The allocation of overall uncertainty to the emission factor
and activity data components has been made for illustrative purposes onlyon the basis of judgement by the IPCC/OECD technical staff. These values
should not be used for estimating uncertainty for a particular nationalinventory. They are provided to assist users of the Guidelines to consider
relative uncertainties in the basic science underlying different componentsof their inventories.

Revised 1996 IPCC Guidelines for National Greenhouse Gas Inventories:

Reporting Instructions

A1.3MANAGING UNCERTAINTIESTABLE A1-1
UNCERTAINTIES DUE TO EMISSION FACTORS AND ACTIVITY DATA
12345Gas

Source category

Emission factor

Activity data

Overall uncertainty

UEUAUT

CO2Energy

7%

7%

10%

CO2

Industrial
Processes

7%

7%

10%

CO2

Land Use Changeand Forestry

33%

50%

60%

CH4,

Biomass Burning

50%

50%

100%

CH4

Oil and Nat. Gas
Activities

55%

20%60%

CH4

Coal Mining andHandling Activities

55%

20%60%

CH4

Rice Cultivation

3/4

1/4

1CH4Waste

2/3

1/3

1

CH4Animals

25%

10%

25%

CH4

Animal waste

20%

10%20%

N2 O

IndustrialProcesses

35%35%50%

N2 O

Agricultural Soils

2 orders of magnitude

N2 O

Biomass Burning

100%

Note: Individual uncertainties that appear to be greater than ± 60% are not shown. Instead judgement as to the relative
importance of emission factor and activity data uncertainties are shown as fractions which sum to one.

A1.2 Procedures for Quantifying
Uncertainty
Estimating Uncertainty of Components
To estimate uncertainty by source category and gas for a national inventory,
it is necessary to develop information like that shown in Table A1-1, but
specific to the individual country, methodology and data sources used. In
scientific and process control literature the 95 per cent (± 2) confidence
limit is often regarded as appropriate for range definition. Where there issufficient information to define the underlying probability distribution forconventional statistical analysis, a 95 per cent confidence interval should becalculated as a definition of the range. Uncertainty ranges can be estimatedusing classical analysis (see Robinson) or the Monte Carlo technique (inEggleston, 1993). Otherwise the range will have to be assessed by nationalexperts.

A1.4Revised 1996 IPCC Guidelines for National Greenhouse Gas Inventories:

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If possible ranges should be developed separately for•emission factors (and other assumptions in the estimation method)(column 3 of Table A1-1).

•

socio-economic activity data (column 4 of Table A1-1)

Combining Uncertainties
It is necessary to derive the overall uncertainty arising from the
combination of emission factor and activity data uncertainty. IPCC/OECD
suggest that emission factor and activity data ranges are regarded asestimates of the 95 per cent confidence interval, expressed as a percentageof the point estimate, around each of two independent components (either
from statistically based calculations or informal ex ante judgements).
On this interpretation (for quoted ranges extending not more than 60 per
cent above or below the point estimate) the appropriate measure of overall
percentage uncertainty UT for the emissions estimate would be given by thesquare root of the sum of the squares of the percentage uncertainties
associated with the emission factor (UE) and the activity data (UA). That is,for each source category:
UT = ±(U + U ) ; so long as U , U < 60% 12E2AEA

For individual uncertainties greater than 60 per cent the sum of squares
procedure is not valid. All that can be done is to combine limiting values todefine an overall range, though this leads to upper and lower limiting values
which are asymmetrical about the central estimate2.
Estimated total emission for each gas is of course the summation Σ Ci
where Ci is the central estimate of the emission of the gas in the source
category. The appropriate measure of uncertainty in total emissions inemissions units (not percentages) is then:E = ± (1 / 100) ⋅(∑ U ⋅ C )
2T ,i

2

iwhere UT,i is the overall percentage uncertainty for the source category ofthe gas from Table A1-1. Source categories for which symmetrical limiting
values cannot be defined (because UE or UA exceeds 60 per cent)cannot sensibly be treated in this way. The uncertainty might be handled byreporting that total emissions from gas X are estimated to be Y Mt, ofwhich Y1 Mt had an estimated uncertainty of ± E1 Mt and Y2 Mt had a rangeof uncertainty between - L Mt and + U Mt.

T requires σto be less than about 30% of the central estimate, and we are interpretingthe quoted range as ±2σ

1 The 60% limit is imposed because the rule suggested for U

2 If uncertainties due to the emission factor and the activity data are± E% and ± A% respectively, and the upper and the lower limits of overall
uncertainty are U% and L% respectively, then U% = (E+A+E.A/100) andL% = (E+A-E.A/100).

Revised 1996 IPCC Guidelines for National Greenhouse Gas Inventories:

Reporting Instructions

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MANAGING UNCERTAINTIES

A1.3 Implications
If the assumptions in Table A1.1 are correct then typical uncertainties innational emissions estimates range between:•± 10% for CO2 from fossil fuels although this may be lower for somecountries with good data and where source categories are well defined(IPCC, 1993; von Hippel et al., 1993)• ± 20% and ± 100% for individual methane sources (though the overallerror might be ± 30%)
• perhaps two orders of magnitude for estimates of nitrous oxide fromagricultural soils
These uncertainties will affect the level of quantitative understanding of
atmospheric cycles of greenhouse gases that can be derived using thesummation of inventories.
The situation is less critical for monitoring emissions mitigation options,because the profile of the emissions time series will be relatively insensitive
to revisions to the emissions estimation methodology. However very
different levels of uncertainty for different gases will be inevitable for sometime to come, and this will need to be recognised in any move towards acomprehensive approach to greenhouse gas mitigation.

A1.4 References(IPCC) Intergovernmental Panel on Climate Change (1992), Climate Change
1992: The Supplement to the IPCC Scientific Assessment.
The method for combining errors in a multiplicative chain are given in manystatistical textbooks, but note Jennifer Robinson's discussion (Onuncertainty in the computation of global emissions from biomassburning, Climatic Change, 14, 243-262) about the difficulties whicharise at high coefficients of variation.
H S Eggleston (1993), “Uncertainties in the estimates of emissions of VOCsfrom Motor Cars.” Paper presented at the TNO/EURASAP
Workshop on the Reliability of VOC Emission Databases, June 1993,
Delft, The Netherlands.
IPCC (1993), “Preliminary IPCC national GHG inventories: in depthreview.” Report presented at the IPCC/OECD Workshop on
National GHG Inventories, October 1993, Bracknell, UK.
von Hippel et al. (1993), “Estimating greenhouse gas emissions from fossil
fuel combustion”, Energy Policy, 691-702, June 1993.

A1.6

Revised 1996 IPCC Guidelines for National Greenhouse Gas Inventories:

Reporting Instructions