Third IMO Greenhouse Gas Study 2014

82 Third IMO GHG Study 2014
1.4.4 Comparison of top-down and bottom-up inventories
Four main comparators are essential to understanding the similarities, differences and joint insights that derive
from the top-down and bottom-up inventories:
1 estimates of fuel totals (in million tonnes);
2 allocation of fuel totals by fuel type (residual, distillate and natural gas, or HFO, MDO and LNG as
termed in this study);
3 estimates of CO 2 totals (in million tonnes), which depend in part upon the allocation of different fuel
types with somewhat different carbon contents;
4 allocation of fuel totals as international and not international (e.g. domestic and fishing).
Given the results presented in Sections 1.1.3 and 1.3.2, there is a clear difference between the best estimates of
the top-down and bottom-up methods. This difference has been documented in the scientific peer-reviewed
literature and in previous IMO reports. This study finds that the best estimates of fuel consumption differ by
varying quantities across the years studied. Smaller differences between top-down and bottom-up total fuel
consumption are observed after the availability of better AIS coverage in 2010. However, in all cases, the
activity-based bottom-up results for all fuels are generally greater than the top-down statistics.
a) All marine fuels b) International shipping
Figure 61: Top-down and bottom-up comparison for a) all marine fuels and b) international shipping
Allocation of fuel inventories by fuel type is important and comparison of top-down allocations with initial
bottom-up fuel type results provided important QA/QC that helped reconcile bottom-up fuel type allocation.
The fuel split between residual (HFO) and distillate (MDO) for the top-down approach is explicit in the
fuel sales statistics from IEA. However, the HFO/MDO allocation for the bottom-up inventory could not be
finalized without consideration of top-down sales insights. This is because the engine-specific data available
through IHSF are too sparse, incomplete or ambiguous with respect to fuel type for large numbers of main
engines and nearly all auxiliary engines on vessels. QA/QC analysis with regard to fuel type assignment
in the bottom-up model was performed using top-down statistics as a guide together with fuel allocation
information from the Second IMO GHG Study 2009. This iteration was important in order to finalize QA/QC
on fuel-determined pollutant emissions (primarily SO x and PM), and results in slight QA/QC adjustments for
other emissions. Figure 62 presents a side-by-side comparison of top-down, initial and updated bottom-up
approaches to fuel type allocations.