Third IMO Greenhouse Gas Study 2014

48 Third IMO GHG Study 2014
Figure 31: Summary graph of annual fuel use by international shipping,
estimated using the top-down and bottom-up methods
Particular care must be taken when interpreting the domestic fuel consumption and emissions estimates from
both the top-down and the bottom-up methods. Depending on where domestic shipping and fishing buys its
fuel, it may or may not be adequately captured in the IEA marine bunkers. For example, inland or leisure and
fishing vessels may purchase fuel at locations that also sell fuel to other sectors of the economy and therefore
be misallocated. In the bottom-up method, fuel consumption is included only for ships that appear in the
IHSF database (and have an IMO number). While this should cover all international shipping, many domestic
vessels (inland, fishing or cabotage) may not be included in this database. An indication of the number of
vessels excluded from the bottom-up method was obtained from the count of MMSI numbers observed on
AIS but for which no match to the IHSF database was obtained. The implications of this count for both the
bottom-up and top-down analysis are discussed in Section 1.4.
1.3.3 Trends in emissions and drivers of emissions 2007–2012
Figures 32–37 present indexed time series of the total CO 2 emissions for three ship types – oil tankers, container
ships and bulk carriers – during the period studied. The figures also present a number of key drivers of CO 2
emissions estimated in the bottom-up method that can be used to decompose CO 2 emissions trends:
• the total CO 2 emissions are a function of the total number of ships and average annual fuel consumption;
• the average annual fuel consumption is primarily a function of days at sea and the extent of adoption
of slow steaming;
• all trends are indexed to their values in 2007.
These drivers of average annual fuel consumption can also be influenced by changes in the average specification
of the fleet (average capacity, average installed power, etc.). These are of less significance than the key trends
of speed and days at sea.
The contrast between the three plots shows that these three sectors of the shipping industry have changed
in different ways over the period 2007–2012. The oil tanker sector reduced its emissions by a total of 20%.
During the same period the dry bulk and container ship sectors also saw absolute emissions reductions but by
smaller amounts. All ship types experienced similar reductions in average annual fuel consumption, but the
difference in fleet total CO 2 emissions is explained by the combination of these reductions with differences in
the number of ships in service. The reduction in average days at sea during the period studied is greatest in the