MnrAq

Inventories of CO2 emissions from international shipping 2007–2012 79
Year
Table 26: Observed, unobserved and active ship counts (2007–2012)
Total
Transport ships
% in service
% of in-service
ships observed
on AIS
Total
Non-transport ships
% in service
% of in-service
ships observed
on AIS
2007 58,074 89% 62% 49,396 99% 19%
2008 59,541 89% 66% 50,704 98% 24%
2009 61,065 90% 69% 50,872 100% 29%
2010 69,431 83% 68% 52,941 98% 31%
2011 72,462 75% 69% 51,961 96% 32%
2012 60,670 93% 76% 54,077 96% 42%
There is a large discrepancy between the number of AIS-observed and in-service ships, with fewer in-service
ships appearing on AIS than would be expected. This discrepancy is far greater for non-transport ships but still
significant for transport ships. Explanations for this discrepancy include:
• a large number of ships classified as in service were not actually so;
• the AIS transponders of in-service ships were not turned on during the year or were faulty/sending
spurious signals;
• ships were out of range of any AIS receiving equipment (shore-based or satellite).
The maps of AIS coverage shown in Annex 3 demonstrate that the third explanation (out of range) is plausible
for the shortfall in the earlier years (2007, 2008 and 2009). However, the consistency in the shortfall between
the number of observed ships and in-service ships across the years (particularly from 2010 onwards, when
satellite AIS data are available) does not support this as the only explanation.
Table 27 lists the statistics for four ship types: bulk carriers, container ships, general cargo ships and oil tankers.
For these fleets, which account for the majority of shipping emissions, the percentage of in-service ships that
also appear in AIS is generally excellent (90%–100%), although there are some notable exceptions. Only 50%
or less of the smallest size category of oil tankers, bulk carriers and general cargo ships are observed on AIS,
regardless of the year and the quality of AIS coverage.
This implies that the quality of AIS coverage for the ships most important to the inventory is good, but that
there are shortcomings in the quality of either the AIS coverage or the IHSF database for the smallest ship
size categories. Even as the geographical coverage of the AIS database increases over time, there are many
ship types and sizes for which the percentage of in-service ships observed in AIS reduces over time (this is
particularly true of the larger container ships and bulk carriers). This trend is indicative of deterioration in the
quality of the IHSF status indicator since 2007, 2008 and 2009.
The average duration period for ships that are observed
Table 27 also describes the percentage of the year for which there is a reliable estimate of activity for ships
observed on AIS. (The method and judgement of reliability are described in detail in Annex 3.) Consistent with
the switch from solely shore-based AIS in 2007, 2008 and 2009 to shore-based and satellite AIS in the later
years, there is a substantial increase over the period of this study in the percentage of the year for which a ship
can be reliably observed from its AIS transmissions. Many of the smaller ship categories are well observed
even in the early years of this study, which is indicative of the ships being operated in coastal areas of land
masses where there was good shore-based AIS reception (e.g. particularly Europe and North America).
A composite of the number of ships observed and the duration for which they are observed can be found by
taking the product of the two statistics in Table 27:
% total in-service coverage = % in-service ships on AIS × % of the year for which they are observed
Figure 59 displays the trend over time of the percentage of total in-service coverage for four of the fleets
sampled in Table 27. As expected from the increased geographical coverage of AIS data with time, the total
in-service coverage increases. In 2012, the average in-service large container or bulk carrier can be reliabily
observed in the AIS data set assembled by this consortium for nearly 70% of the time. Coverage of the largest
bulk carriers nearly tripled between 2010 and 2012, showing that rapid improvements have been observed