Engine emissions have been regulated by the IMO under MARPOL Annex VI since 2005 when the 1997 Protocol adding the annex came into effect.
In the initial wording of Annex VI there was no mention whatsoever of greenhouse gases (GHGs) and it would be another six years before the IMO adopted the efficiency measures such as EEDI and ship energy efficiency management plans (SEEMP) which aimed at reducing CO2 emissions.
The volume of goods transported by sea has increased from 4.8 billion tonnes to 11 billion tonnes between 1996 and 2021, according to UNCTAD
The first actions on GHGs was limited to reductions in CO2 and did not occur until 2015 when Phase 1 of EEDI became effective for ships over 400GT.
The first IMO GHG Study in 2000 estimated that in 1996 shipping contributed 1.8% of the world total anthropogenic CO2 emissions. That increased to 2.7% in 2007 (2nd IMO GHG Study), 2.2% in 2012 (3rd GHG Study) and 2.0% in 2018 (4th GHG Study). Over the same period UNCTAD reports the volume of goods transported by sea increased from 4.8 billion tonnes to 11 billion tonnes.
To date only CO2 emissions have been regulated by way of the EEDI and more recently the EEXI for older ships. However, the IMO’s ambition as set out in the 2023 IMO Strategy on Reduction of GHG Emissions from Ships adopted at MEPC 80 now includes targets for all GHG emissions.
The 2023 GHG Strategy foresees a basket of measures to be developed. These cover a technical element, namely a goal based marine fuel standard regulating the phased reduction of the marine fuel’s GHG intensity and an economic element, on the basis of a maritime GHG emissions pricing mechanism.
A timeline for the strategy to be implemented runs through to MEPC 83 in Spring 2025 for the measures to be finalised followed by an extraordinary one or two-day MEPC (six months after MEPC 83 in Autumn 2025) for adoption of the measures and then continuing on with entry into force in 2027.
The switch from CO2 reduction to GHG reduction is a reflection of the increasing use of LNG and the introduction of methanol, ammonia and other alternatives over time. As well as CO2, future IMO regulation will be around methane slip from using LNG as a fuel and Nitrous Oxide N2O – not to be confused with NOx – potentially an issue with ammonia burning engines, and black carbon.
It is methane slip that is the most pressing issue due mostly to the increasing number of ships other than LNG carriers being built with dual-fuel engines. Methane slip is the release of unburnt fuel to the atmosphere and is aproblem because methane has a global warming potential (GWP) more than 25 times that of CO2.
Assuming complete combustion, LNG produces around 25% less CO2 compared to oil fuels for the same propulsive power measured tank to wake but it is recognised that early four stroke engines and Otto cycle two-stroke engines suffered significantly from methane slip. In newer engines this is being addressed and slip rates have been cut by over 90% since the late 1990s.
N2O is not produced in significant quantities from the current crop of marine engines and fuels but could become more problematic if ammonia becomes a mainstream fuel. This is because it has a very high GWP 265 times that of CO2. Engine makers are fully aware of the issue and are designing N2O reduction into the ammonia engines now being developed.
Deciding on the technical elements for controlling both N2O and methane will be challenging. The conditions under which N2O is produced vary with both loading and combustion temperatures so do not correlate with quantity of fuel used. As methane slip only occurs on dual-fuel engines when running on gas, it would be difficult to introduce it into the EEDI formula as it will be an unknown where the engine is intended to operate on both LNG and an oil fuel.
GWP Potential of Different Gases
Greenhouse gases do not all have the same characteristics as each other. Some absorb more energy than others and they decay in the atmosphere at diff erent rates.
To allow comparisons of their global warming impact, Global Warming Potential (GWP) was developed to measure how much energy one ton of a gas will absorb over a given period of time relative to a ton of CO2. The IPCC uses both 100 and 20 year time scales in its regular Assessment Reports with the 100 year scale being used as the standard under the Paris Agreement.
The GWP of different gases varies over 20 year and 100 year timescales.
The IPCC’s method of calculation has varied over time and also diff ers from some other measures’ calculations. This is why equivalence figures may vary in reports and communications. The IMO’s figures in the 4th GHG Study use the non-feedback figure from IPCC AR5. The IPCC’s AR6 report dropped the non-feedback measure and the feedback included figures are slightly lower than the earlier report
