Third IMO Greenhouse Gas Study 2014

Inventories of CO2 emissions from international shipping 2007–2012 83
a) Top-down fuel type allocation
b) Initial bottom-up allocation c) Updated bottom-up allocation
Figure 62: Comparison of top-down fuel allocation with initial and updated bottom-up fuel allocation
(2007–2012)
Figure 62 a) and c) show that relative volumes of residual to distillate marine fuel (HFO to MDO) are similar.
This is because the updated allocation in the bottom-up inventory is constrained to replicate the reported IEA
fuel sales ratios. The year-on-year allocations are also constrained by bottom-up analysis that identifies vessel
categories with engines likely to use distillate fuel. A further constraint is that an MDO assignment applied to
a vessel category in any year requires that MDO be assigned to that category in every year.
The CO 2 comparison corresponds closely to the total fuel values, with the exception of the LNG consumption
identified in the bottom-up inventory. The IEA statistics report zero international marine bunkers of natural
gas (LNG), as shown in Table 9 in Section 1.1.3. Trends in CO 2 emissions are nearly identical to total fuel
estimates, with negligible modification by the fuel type allocation. Trends in the top-down inventory suggest
a low-growth trend in energy use by ships during the period 2007–2012. This is consistent with known
adaptations and innovations in the international shipping fleet to conserve fuel during a period of increasing
energy prices and global recession.