MnrAq

234 Third IMO GHG Study 2014
Figure 23: Stacked graph showing sum of fuel transfer balance
and export-import discrepancy
Only qualitative assumptions on the possible percentage of accuracy within the marine sectors can be attempted
based on the available literature. Le Quéré et al. (2009) used an uncertainty in CO 2 emissions of ±6% for global
inventories, but that necessarily means that some sectors and nations can have greater than 6% uncertainty,
especially smaller sectors; conversely, small percentage uncertainties in energy-consuming nations or sectors
may represent very large volumes of fuel. Marland (2008) reported that these types of uncertainties and errors
showed “no systematic bias, and the global totals were very similar”. Relative differences were largest for
countries with weaker national systems of energy statistics, and absolute differences were largest for countries
with large emissions. Again, this literature did not assess marine fuel statistics specifically, but reported on
overall energy balance integrity. Based on the literature, we cannot quantify the remaining accuracy of marine
fuel consumption from top-down statistics.
Results of top-down uncertainty analysis
We present a modified estimate of top-down marine fuels totals by adding the fuel volumes attributed to
export-import discrepancies for fuel oil and gas diesel and by adding the additional fuel volumes associated
with the positive balance of world fuels transfers. These represent the primary and secondary sources of
quantified uncertainty. We add these volumes to the sum of reported fuel sales for fuel oil and gas diesel, to
assess the total additional fuel that may be considered part of the shipping demand for energy. Our logic in
combining known and reported marine fuel consumption by international shipping, domestic shipping and
fishing is as follows:
1 The uncertainty in allocation of marine fuels among international voyages, domestic shipping and
fishing remains unquantified; we therefore produce an assessment of uncertainty in top-down
estimates that is independent of the allocation uncertainty challenge.
2 The total marine fuels volumes reported in the Second IMO GHG Study 2009 included such a
combined statistic, the consensus estimate for bounding 2007 bottom-up fuel consumption; our
analysis is therefore consistent with that study.
3 This general summary of the quantified uncertainty in top-down fuel consumption serves important
comparison tasks in this scope of work.
Figure 24 presents a time series of the quantified change in top-down fuel consumption by represented world
net export-import discrepancies and world net fuel transfers balances in addition to the reported marine fuel
totals for 1971–2011. Figure 25 and Table 16 present these results for 2007–2011.