Use of Models and Facility-Level Data in Greenhouse Gas Inventories

Use of Models and Facility-Level Data in Greenhouse Gas Inventories
Why is inter-annual variability included in estimates?
The 2006 IPCC Guidelines have an implicit goal that the methods used to create estimates provide for annual estimation.
The following section is on page 2.11 of Chapter 2, Volume 1 of the 2006 Guidelines, in section 2.2.3, Adapting data for
inventory use.
Multi-year averaging: Countries should report annual inventory estimates that are based on best
estimates for actual emissions and removals in that year. Generally, single year estimates provide the
best approximation of real emissions/removals and a time series of single year estimates prepared
according to good practice can be considered consistent. Countries should, where possible, avoid
using multi-year averaging of data that would result in over- or under-estimates of emissions over
time, increased uncertainty, or reduced transparency, comparability or time-series consistency of the
estimates. However, in some specific cases that are described for specific sectors in Volume 2-5,
multi-year averaging may be the best or even the only way to estimate data for a single year. In the
case of high or uncertain annual variability – as in the growth of various tree species in a year – and
where there is higher confidence in the average annual growth rate over a period of years then multiyear
averaging can improve the quality of the overall estimate.
The methods considered as good practice may (1) derive averages of multi-year estimates, (2) reflect inter-annual
variability in only activity data, that is, for the human caused but not the biological variation, and (3) may fully reflect, on
an annual basis, inter-annual variability in both human activity rates and biological variations due to externalities such as
climate.
The way that estimates reflect inter-annual variability is therefore more to do with method chosen and measurement
frequency than to do with estimation Tier. That said, Tier 3 systems more typically reflect the conjoint inter-annual
variability effects of human activity and biological variations. Tier 3 models do not create inter-annual variability, they just
typically provide more of a reflection of the actual inter-annual variability than Tier 3 measurement methods that use
multi-year averaged inputs and reflect multi-year averaged emissions estimates.
Forms of multi-year averaging that mask inter-annual variability
Input data (activity and/or stock change/emissions estimates) can be subjected to multi-year averaging. This can be
brought about by either extended measurement periods (including observations/survey, etc.) or through averaging of
multiple annual measurements. While IPCC methods provide for this multi-year averaging of input data, and
recommend it in some instances, there are no provisions for multi-year averaging of resultant emissions estimates.
While the reporting of actual annual estimates, and consequent inter-annual variability, is consistent with an overall goal
of actual annual estimates, the multi-year averaging of input data would appear to detract from this goal.
For activity data, multi-year averaging could be brought about by:
• statistical collections, surveys etc., being conducted or reported over multiple years;
• observations, e.g., satellite data, being at a frequency exceeding a single year.
For stock changes/emissions estimates, multi-year averaging of input data can be achieved by:
the use of emissions factors that remain static year-by-year;
• measurement programs with a return frequency of more than one year;
• use of empirical models (not driven by climate) that average responses over time; and
• use of models (driven by climate) but using long-term (greater than one year) averaged climate.
Is averaging input data the same as averaging resultant emissions estimates?
The answer to this is, probably not, but is dependent on how the input data is used. For the simplest of algorithms, the
averaging of inputs may not impact on total emissions reported, but will of course impact on the time trend of emissions.
This is not a concern if only the long-term total emissions are of interest. However, for reporting over short time periods
this could be problematic (see Figure 3 below).
IPCC Expert Meeting Report 36 TFI