MnrAq

Inventories of CO2 emissions from international shipping 2007–2012 39
2 assign fleet sectors to domestic service and subtract from fleet;
3 combine T-D heuristics and fleet sector information to match the vessel types most likely to serve
domestic shipping (bottom-up) with expectations of total fraction likely to use domestic bunkers
(top-down).
The Second IMO GHG Study 2009 used method 3, a combined application of the top-down heuristic
and removal of some vessel types. However, the study noted significant uncertainties with this approach.
Specifically, it assumed that ship activity was proportional to data on seaborne transport. The study noted
that, over the course of a year’s activity, a given vessel could be engaged in both international shipping
and domestic navigation. “Since the [Second IMO GHG Study 2009] activity-based model cannot separate
domestic shipping from international shipping, figures from bunker statistics for emissions from domestic
shipping [were] used in the calculation of emissions from international shipping” (Second IMO GHG Study
2009, paragraph 3.17). This study explicitly removed fleet sectors associated with fishing, fixed offshore
installations (production vessels) and domestic navigation relying on fuel totals reported in their top-down
analysis based on IEA statistics.
The Third IMO GHG Study 2014 consortium chose not to apply allocation methods 1 or 3 and selected
method 2, for several reasons. Method 1 requires a simplistic and arbitrary direct application of the top-down
fuel ratios to bottom-up totals. The main disadvantage of method 1 is that it can be applied to the inventory
total only; results cannot be tied to bottom-up insights within vessel categories. A related disadvantage is
that the assumption may be untestable, preventing direct quality assurance or control and disabling any
quantitative consideration of uncertainty.
Allocation method 3 requires subjective judgements to be imposed on the bottom-up data beyond a testable
set of assumptions applied to vessel types. For example, the 2009 study imposed additional definitions of
oceangoing and coastwise shipping, designating some fleet sectors like cruise passenger ships, service and
fishing vessels and smaller ro-pax vessels as coastwise. However, that study did not reconcile or discuss
whether the fuel totals allocated to coastwise vessels corresponded to an international versus domestic
determination within its activity-based method. Moreover, an attempt to determine which shipping was
coastwise, as opposed to transiting along a coastal route, was beyond scope of the study.
The Third IMO GHG Study 2014 applies allocation method 2 with information provided in AIS to support
the bottom-up methodology. Based on general voyage behaviour, some ship types are likely to engage in
international shipping more often than domestic navigation. These types include transport and larger ferry
vessels, as listed in Table 12. This allocation, therefore, also identifies ship types that can be expected to
engage mostly in domestic navigation, including non-transport vessels, such as offshore and service vessels,
yachts and smaller regional ferry vessels (see Table 13). Results using allocation method 2 allow comparison
between bottom-up and top-down allocation of international shipping and domestic navigation. As a caveat,
method 2 might overestimate international shipping and could increase uncertainty, which is discussed in
Sections 1.4 and 1.5.