The emissions are in the detail: the IMO must account for shipping’s full climate impact
For shipping to deliver the meaningful emissions reductions urgently required, the industry must be held accountable for its full climate impact. In the upcoming International Maritime Organization negotiations, Member States must account for all of the sector’s emissions in any emissions reduction measures agreed.
In 2023, Member States at the International Maritime Organization (IMO) came to a landmark decision, agreeing the 2023 IMO Strategy on Reduction of GHG Emissions from Ships. Now, 2025 is primed to mark another such watershed moment for the shipping industry, currently responsible for around 3% of global greenhouse gas (GHG) emissions every year: in a series of negotiations, the first of which started this week, IMO Member States are set to adopt ‘a basket of mid-term measures’ to help the industry meet the 2023 Strategy decarbonisation targets in line with the principles of a just and equitable transition.
Slated to be finalised in April and ratified in October, these measures will include a technical element in the form of a goal-based marine fuel standard (GFS), and an economic element, such as a GHG emissions pricing mechanism. The details are yet to be decided, including crucially how the scope of GHG emissions covered by each element should be calculated. This calculation will be a key determinant of how far the measures will go toward successfully reducing emissions.
Greenhouse gas fuel intensity at the IMO
When marine fuels are burned, the chemical energy stored in the fuel is converted to thermal energy, which is used to propel the ship. As the fuel combusts, it may release GHGs as a waste product. Greenhouse Gas Fuel Intensity (GFI) is what we use to measure the amount of GHGs emitted per unit of energy used.
Both the technical and economic elements of the proposed basket of measures partly rely on GFI calculations. But the jury’s still out on how it will be calculated. Currently, two methods have been proposed: a Well-to-Wake (WtW) calculation, and an adjusted Tank-to-Wake (TtW) calculation.
The main difference between these two proposals is exactly which emissions are accounted for.
What is Well-to-Wake (WtW)?
The proposed WtW calculation accounts for emissions which occur after fuel is added to the ship’s tank, as well as upstream emissions involved in the fuel’s extraction, production, storage, and transport. All emissions which occur during the fuel lifecycle are therefore counted.
What is Tank-to-Wake (TtW)?
On the other hand, a simple TtW approach would only count emissions which occur after the fuel enters the ship’s tank. The “adjusted TtW” proposal on the table at the IMO, its proponents claim, uses this simple TtW approach as a base, but accounts for additional upstream emissions using an adjustment.
Whether or not the adopted measures will account for the full climate impact of the shipping industry weighs largely on which method is used.
The case of Ammonia’s greenhouse gas emissions
One of the suggested frontrunners for alternative shipping fuels, ammonia, provides a sound example of the basic difference between WtW and TtW calculations in practice.
A simple TtW calculation for ammonia would measure just the GHG emissions released when the fuel is burnt onboard a ship. These emissions are primarily nitrous oxide (N2O), a potent GHG with a climate impact 273 times larger than CO2 on a 100-year timescale. N2O can form during ammonia combustion, and can also form indirectly if reactive nitrogen species such as ammonia, nitric oxide (NO) and nitrogen dioxide (NO2) (collectively known as NOx) leak, spill or are released into the environment. Such leakages also pose environmental and health risks. The N2O emissions from ammonia as a shipping fuel remain uncertain, and are critical for deciding the extent to which ammonia can deliver emissions reductions.
But even before this point, significant emissions will have already been released in ammonia’s lifecycle, and these are left out of a simple TtW calculation. Ammonia is produced by the Haber-Bosch process which reacts nitrogen with hydrogen. There are various ways of conducting this reaction, including:
“Brown/grey ammonia” is produced when the hydrogen is extracted from coal (brown) or natural gas (grey), which is then reacted with nitrogen. All reactions are powered by electricity generated by fossil fuels.
“Green ammonia” is produced using electrolysis of water to produce hydrogen, with reactions powered by electricity from renewable sources.
Most ammonia today is grey rather than green. Even though any colour ammonia produces the same emissions at the point of combustion, brown and grey ammonia have significantly higher emissions during production from the fossil fuels used for hydrogen and electricity. These latter emissions do get factored into WtW GHG emissions but are ignored by TtW calculations. Put more simply:
Brown/grey and green ammonia have the same TtW emissions, those produced when burnt on the ship;
Brown/grey ammonia has significantly higher WtW emissions than green ammonia, owing to the use of fossil fuels during their production.
What does this mean for the IMO measures?
It is for this reason that a simple TtW calculation can be dangerously misleading, implying that brown/grey and green ammonia have the same climate impact, despite their very different lifecycle GHG emissions.
What about the “adjusted TtW” proposal? In this method, GFI values are calculated (warning: incoming maths) by reducing the WtW emissions by a certain percentage. This percentage would be the ratio of the average TtW and WtW emissions for international shipping in 2008. So, for example, if the 2008 WtW emissions for the average fuel were 100 gCO2e/MJ and the TtW emissions were 80 gCO2e/MJ, the GFI for any given fuel would be calculated by adjusting its WtW emissions by 80/100, or a factor of 0.8. The effect of this is that the adjusted TtW emissions for a fuel would be 20% lower than the WtW emissions.
If the maths is tricky to get your head around, don’t worry – the bottom line is that this approach may be problematic for two reasons:
Emissions released during a fuel’s production are an important part of the shipping industry’s overall climate impact. Using the adjusted TtW approach would exempt the industry from responsibility for a significant percentage of its emissions.
Marine fuel use in 2008 was dominated by fossil fuels. Therefore, applying this fixed reduction factor to non-fossil fuels such as ammonia will scale their GFI values as though they were fossil fuels. The resulting GFI values are ultimately arbitrary, and unlikely to accurately represent TtW emissions from these alternative fuels.
The graph below shows just how much lower GFI values from the adjusted TtW method could be compared to the climate impact reflected by WtW emissions. It also shows how the “adjusted TtW” calculation can lead to highly misleading GFI values for ammonia or other, non-conventional marine fuels.
Figure 1. Calculated GFI values (y axis) using a WtW (black line) and adjusted TtW (orange line) method, plotted against actual WtW GHG emissions (x axis).
Why is it so important to account for ALL shipping’s emissions?
In short, neither the simple nor adjusted TtW calculation accurately reflects marine fuels’ total climate impacts. As we saw with ammonia, some marine fuels involve high emissions during production – these must be properly accounted for to ensure that the basket of measures delivers meaningful emissions reductions. This can only be captured by GFI values using a WtW approach.
What may seem like a petty detail is actually extremely important for the IMO’s midterm measures. To decarbonise the shipping sector with the urgency needed means accounting for all shipping’s emissions, including upstream emissions.
It is essential that Member States use these negotiations to advocate for ambitious and robust measures based on WtW GFI values. Only by doing so can they truly drive a just and equitable transition for the shipping industry.
Find out about our work at the IMO and what we want to see from negotiations.
