MnrAq

Inventories of CO2 emissions from international shipping 2007–2012 87
Figure 63: Adjusted marine fuel sales based on quantitative uncertainty results (2007–2011)
1.5.2 Bottom-up inventory uncertainty analysis
Bottom-up uncertainty in this study is conditioned on the quality control of information for specific vessels,
application of known variability in vessel activity to observed vessels within similar ship type and size fleets
and the way in which activity assumptions are applied to unobserved vessels within similar ship type and size
fleets. In other words, the quantification of uncertainty is linked to the quality control section of this report.
One of the most important contributions of this study in reducing uncertainty is the explicit quality control
to calculate fuel use and emissions using specific vessel technical details; this directly accounts for variability
within a fleet bin, and replaces the uncertainty with the average technical parameters in the Second IMO
GHG Study 2009 calculations with the average technical parameters. Another important contribution to
reducing uncertainty is the direct observation of activity data for individual vessels, i.e. speed and draught
aggregated hourly, then annually.
Figure 64 presents the uncertainty ranges around the top-down and bottom-up fuel totals for the years studied.
The vertical bars attached to the total fuel consumption estimate for each year and each method represent
uncertainty. This study estimates higher uncertainty in the bottom-up method in the earlier years (2007, 2008
and 2009), with the difference between these uncertainty estimates being predominantly attributable to the
change in AIS coverage over the period of the study. The uncertainty in the earlier years is dominated by
uncertainty in the activity data, due to the lack of satellite AIS data. In later years (2010, 2011 and 2012), this
uncertainty reduces, but the discrepancy between the number of ships identified as in service in IHSF and the
ships observed on AIS increases (relative to the earlier years). The result is that the total bottom-up uncertainty
only reduces slightly in the later years when improved AIS data is available.
The top-down estimates are also uncertain, and include observed discrepancies between global imports and
exports of fuel oil and distillate oil, observed transfer discrepancies among fuel products that can be blended
into marine fuels and the potential for misallocation of fuels between sectors of shipping (international,
domestic and fishing).