Third IMO Greenhouse Gas Study 2014

236 Third IMO GHG Study 2014
particularly since the 1980s. This is evident in Figure 24, where the percentage of adjustment due to exportimport
discrepancies never exceeded 10% prior to 1980, but always exceeded 10% after 1980. In fact, the
percentage adjustment due to export-import allocation uncertainty has never been lower than 22% since
1982. More recently, Table 16 and Figure 25 illustrate the top-down adjustment for 2007–2011. During these
years, the average adjustment due to export-import allocation uncertainty averaged 28%.
The secondary source of uncertainty, measured by the quantity of adjustment that is supported by our analysis,
derives from the excess balance of fuels that were transferred among domestic consumption sectors in national
inventories. This discrepancy exists because deduction reclassification of energy products in one or more fuel
sectors remains undocumented as an addition reclassification in another sector. In other words, fuel-transfer
deductions appear to be blended into marine bunkers to meet ship/engine fuel quality specifications without
accompanying documentation reclassifying them as added to the marine fuel sales volumes. The trend on this
error only slightly increased from the 1970s to mid-1990s, and the magnitude of the error, as a percentage of
marine fuel sales, never exceeded 2% until 1997. Since 1997, the contribution to uncertainty in marine fuel
statistics has more than doubled; nonetheless, during the 2007–2011 period, the average impact on marine
fuel statistics of approximately 3% still remains small compared to export-import allocation uncertainty.
Tertiary sources of uncertainty exist, including different statistical approaches on data collection, reporting
and validation. These have been observed and reported in the Second IMO GHG Study 2009 (see Table 3.1
of that report). Data accuracy is an ongoing QA/QC effort by IEA and others to help minimize these sources of
error and uncertainty. Our work for this update indicates three insights about the nature of uncertainties that
we judge to be tertiary, or smaller than those discussed above.
1 The impact of these uncertainties cannot be shown to be consistently biased; in other words, the sign
of a potential adjustment appears to vary from year to year;
2 Little evidence supports a cumulative effect on marine fuel sales statistics; in other words, the
magnitude cannot be shown to be increasing or decreasing over time; and
3 No uncertainty adjustment can be quantified from the existing statistical differences.
The combined error in recent years associated with these uncertainties ranges from approximately 64 million
tonnes to approximately 87 million tonnes of fuel, as indicated in Table 16 for 2007–2011. Incidentally, the
2007 calculated adjustment would reconcile within 1.2% of the top-down statistics with the activity-based
estimate of 333 million tonnes reported in the Second IMO GHG Study 2009.
Uncertainty in top-down allocations of international and domestic shipping
We anticipate limited ability to evaluate or reduce allocation uncertainty within top-down fuel types. This
could mean that a remaining key uncertainty for IMO will be the designation of top-down marine bunker sales
as domestic or international, without additional empirical data. Options include:
1 Treating reported allocations in existing IEA statistics as certain, and using these to allocate the fuel
adjustments quantified in this uncertainty analysis;
2 Recognizing that allocations in the statistics are also uncertain, and studying ways to adjust both
reported fuel volumes and the adjustments quantified here using the same top-down assumptions,
evidence and conclusions;
3 Treating as independent the marine fuel sales data and the adjustment quantified by uncertainty
analysis using different top-down assumptions, evidence and conclusions; and
4 Coordinating top-down and bottom-up allocation approaches to leverage insights and produce
mutually consistent allocation algorithms.