The CAT derived its historical emissions data for the period 1990 to 2018 from the different IMO GHG reports (IMO - MEPC, 2020; IMO, 2009; Smith et al., 2014).
We use data from the IMO Fourth GHG study for years 2012-2018 (voyage-based approach), extended back to 1990 using growth rates from the IMO 3rd GHG study (2007-2011) and 2nd GHG study (1990-2006) (IMO - MEPC, 2020). We use the estimate based on the voyage-based approach from the IMO Fourth GHG study for the year 2008.
We use growth rates for international shipping CO2 emissions in EDGAR v7 GHG emissions dataset to extend the timeseries to 2021.
This data appears as the solid black line in our graph.
For comparison purposes, we also include emissions calculated using the vessel-based approach (light grey line). These emissions are calculated using the same sources and methods as above. For more on the different approaches, see here.
2030 & 2050 Carbon intensity target
The UMAS study, which forms the top end of our policy range (see below), provides the percentage reduction below 2008 levels using the AER vessel-based approach (Smith, Galbraith, et al., 2022). We calculate the carbon intensity for both 2030 and 2050 from these reduction values and data for the AER vessel-based approach in the IMO’s Fourth Study for 2008.
To convert between vessel and voyage, we use the ratio between AER (vessel)/(voyage) in 2018 (that last year for which we have data).
We assume that the differences between our current policy emission projections and the UMAS study are due to differences in transport work assumes and thus that the carbon intensity does not change.
We calculate the implied transport work in our current policy projections using the carbon intensity value derived from the UMAS study and apply it to the target carbon intensity values to estimate CO2 emissions in 2030 and 2050.
2050 Absolute Target
The 2050 target applies to all GHG emissions (excluding black carbon); however, we calculate CO2 emissions only as this is the basis of our rating system.
We apply the 50% reduction target to 2008 value (voyage-based approach) for CO2 emissions provided in the IMO’s Fourth GHG study.
The Fourth GHG Study provides an estimate for all GHGs (voyage-based) in 2008 using AR5 Global Warming Potential values. To convert to AR4 values (the CAT standard), we assume the same gas ratio as in 2012 as that is the closest year to 2008 for which we have a gas by gas breakdown. We then apply the 50% reduction to derive our emissions estimate.
We calculate CO2 and GHG emission estimates for the 2030 and 2040 proposed targets using the same methods as outline above for the existing 2050 absolute target.
Current policy projections
The top end of our current policy projection range is based on a scenario from UMAS (Smith, Galbraith, et al., 2022). The study bases its projections on the SSP2_RCP2.6 L scenario, which corresponds to part of our previous upper bound in our 2021 assessment, but includes the impacts of the pandemic and the latest energy efficient measures. We use the growth rates from this projection and apply these to the last historic year.
The bottom end of our range is based on the minimum values of the three scenarios included in the IMO’s Fourth Study logistics model method (OECD_RCP26_L). To estimate the impact of the latest EE measures, we assume that this scenario will fall in equal proportion to difference between our previous top and bottom ranges and the UMAS study to scale this projection downwards.
Emissions estimates in the Fourth GHG Study are for all shipping. We take the ratio between all shipping and the voyage-based approach in 2018 to derive voyage-based estimates. The UMAS study uses the vessel-based approach, we assume a constant ratio between vessel and voyage.
Our analysis and rating methodology is currently limited to CO2 emissions.
Scope of sector
The IPCC GHG inventory guidelines divide shipping into international navigation, domestic navigation and fishing (IPCC, 2006). It defines international shipping as “journeys that depart in one country and arrive in a different country”, while domestic shipping includes “journeys from ports to ports within the same country”. International shipping is responsible for most emissions (70-87% in 2018 using voyage or vessel-based approaches respectively) (IMO - MEPC, 2020). Data incorporating both international, domestic shipping and fishing is often referred to as ‘total shipping’.
Our assessment is of international shipping only and all references to shipping should be taken to mean ‘international shipping’ unless otherwise stated.
Emissions from domestic shipping and fishing are considered as part of national totals and are included in the assessment of individual CAT countries.