In July 2023, the International Maritime Organization (IMO) adopted a revised strategy that calls for reducing greenhouse gas (GHG) emissions from ships to net-zero by or around 2050. While the revised strategy is not legally binding, the measures used to implement it can be, and in many ways it’s the stringency of these measures that will ultimately determine shipping’s contribution to future global warming.  

Earlier this week, our colleague highlighted the need for measures that limit emissions from ships measured on a life-cycle basis, the well-to-wake (WTW) emissions. With this blog post, we show how one proposed measure, the GHG Fuel Standard (GFS), can be used to reduce emissions in line with the IMO’s revised 2023 strategy or with a pathway consistent with limiting warming to 1.5°C. 

The GFS being designed now will require ships to use fuels that emit fewer WTW GHG emissions until there is a complete transition to all zero-emission fuels. This GFS is meant to encourage the adoption of new fuels including renewable e-fuels (hydrogen, ammonia, and methanol) and sustainable biofuels; by setting limits on the GHG emissions intensity of fuels, it will drive investments in production capacity and infrastructure for new fuels. One effective design of the GFS would identify the date by which the WTW GHG intensity of marine fuels is to reach zero and include interim GHG intensity targets (at regular intervals) to keep the sector on a steady course toward its final goal. Here we use ICCT’s new Polaris model to estimate the WTW GHG intensity reductions that would be needed to achieve net-zero by 2050 in a pathway consistent with the 2023 IMO GHG strategy. Polaris is a global maritime emissions projection model that reports tank-to-wake (TTW) and WTW emissions as carbon dioxide equivalents (CO₂e) based on the 100-year or 20-year global warming potentials of CO₂, methane, nitrous oxide, and black carbon (we exclude black carbon in this particular analysis because it’s not accounted for in the guidelines on life-cycle GHG intensity of marine fuels). 

Figure 1 shows the straight-line GFS trajectory that satisfies the emissions reduction targets in the 2023 IMO GHG strategy and an S-curve trajectory that would stay below the cumulative emissions limit for 1.5°C estimated here. The GFS trajectories were determined based on the business as usual (BAU) predicted energy use from the Polaris model and target emissions in the 2023 IMO strategy and 1.5°C aligned pathways (using 100-year global warming potentials, GWP100). For 2030, the 2023 IMO strategy set a goal of at least a 20% reduction in absolute GHG emissions compared to 2008 levels, and “striving for” a 30% reduction; for 2040, the GHG reduction goals are at least 70% and striving for 80% below 2008 levels. Predicted energy use from Polaris goes from 10.7 EJ in 2023 to 14.5 EJ in 2050, and we estimated the baseline GHG intensity of marine fuels at 92.5 gCO₂e/MJ from shipping’s fuel mix in 2019 using ICCT’s Systematic Assessment of Vessel Emissions (SAVE) model and excluding black carbon emissions. 

Figure 1. Well-to-wake GHG intensities of marine fuels required to align the IMO GHG Fuel Standard (GFS) with IMO’s 2023 GHG strategy and a 1.5 °C-compatible emissions trajectory.

As Figure 1 illustrates, to achieve the minimum IMO targets, the GHG intensity of marine fuels will have to reduce by 18% to 76 gCO₂e/MJ by 2030 and by 72% to 26 gCO₂e/MJ in 2040 compared to the 2019 baseline. For the “striving” scenario, reductions in 2030 and 2040 will have to be 28% to 67 gCO₂e and 81% to 17 gCO₂e/MJ, respectively. A 1.5°C-aligned pathway requires 32% reductions in WTW GHG intensity in 2030 to 63 gCO₂e/MJ and 99% in 2040 to nearly zero GHG emissions. All pathways require 100% reductions by 2050. Following the GHG intensities in Figure 1 would result in the absolute emissions reduction pathways presented in Figure 2.

Figure 2. Absolute well-to-wake GHG emissions trajectories under each scenario.

Table 1 specifies the GHG intensity limits needed to follow the absolute emissions reduction pathways in Figure 2. This table can be used by policymakers as they develop the GFS.

Table 1. Well-to-wake GHG intensities (gCO₂e/MJ) and reductions in well-to-wake GHG intensities of marine fuels from the 2019 fossil fuel baseline needed to align the GFS with different emissions trajectories.

The cumulative WTW CO₂e emissions compared to “well-below” 2°C (interpreted by us as keeping warming to not more than 1.7°C) and 1.5°C limits are presented in Figure 3. Achieving the minimum or striving IMO targets is consistent with limiting warming to well-below 2°C and the S-curve is consistent with 1.5°C.

Figure 3. Cumulative well-to-wake GHG emissions from 2020-2050 implied by each scenario.

The 2023 GHG strategy also includes a target for the uptake of zero or near-zero GHG emission fuels and/or energy sources that should represent at least 5% (striving for 10%) of the energy used by international shipping by 2030. Achieving even the minimum 5% energy target in 2030 would require 0.6 EJ of zero/near-zero fuels. To put this target into perspective, 0.6 EJ represents around 14% of global biofuel demand in 2022 (~4.3 EJ), whereas shipping (~11 EJ/year) represents about 2.5% of global energy demand (~442 EJ/year). When considered in the context of the limited availability of sustainable advanced biofuels for use in shipping, this underlines the importance of scaling up e-fuels to achieve IMO’s target. 

The stronger the GFS targets, the greater the demand for zero/near-zero GHG emission fuels, the fewer GHGs emitted by the sector, and the greater the likelihood that shipping aligns with both IMO’s GHG strategy and the Paris Agreement. The next opportunity for IMO delegates to contribute to the design of the GFS is at the meeting of the 16th Intersessional Working Group on GHG emissions from ships in March 2024. 

LNG is primarily methane, a powerful GHG that leaks throughout the production and combustion processes—including unburned methane that escapes from marine engines, known as methane slip. As a result, a new ship that’s built to sail on LNG instead of conventional fuels can emit more GHGs on a life-cycle basis, depending on the engine technology and how the LNG is produced.

Figure 1 depicts methane emissions from international shipping in 2021 by ship type and engine type, estimated using ICCT’s Systematic Assessment of Vessel Emissions (SAVE) model (2021 is the most recent year for which we have such data). Liquefied gas tankers, mostly LNG carriers, were the source of 82% of the emissions and were followed by offshore vessels, RoPax ferries, cruise ships, and container ships. Accordingly, the map of methane emissions from LNG-fueled ships in Figure 2 shows they are highly concentrated along LNG trade routes.

Figure 2. Methane emissions from LNG-fueled ships in 2021, aggregated at 0.5 * 0.5 degrees. Sources: Spire (AIS data) and S&P Global (ship characteristics data). This map is without prejudice to the status of or sovereignty over any territory, to the delimitation of international frontiers and boundaries and to the name of any territory, city, or area.

Installations of high-methane-slip LPDF 4-stroke engines are on the rise (Figure 3). More than half of cruise ship capacity by gross tonnage to be built between 2023 and 2025 will run on LNG using these engines, according to IHS Markit (nka S&P Global) data as of July 2023 (the latest available). While a small share of all ships globally, cruise ships have disproportionately high per-ship average emissions because of their hotel and leisure facilities and leaky engines. About the only slightly bright spot here is that the relative share of LPDF 4-stroke engines among engine types in LNG-fueled ships is declining amid growing use of medium-methane-slip LPDF 2-stroke engines that are increasingly used in gas tankers and the low-methane-slip HPDF 2-stroke engines found in most new LNG-fueled container ships and vehicle carriers.

Keep in mind, also, that the real-world methane emissions from ships may be higher than is currently understood. Existing emission inventories, including ours, rely on methane emission factors derived from limited on-board or laboratory measurements of engines. ICCT is leading a project called Fugitive and Unburned Methane Emissions from Ships (FUMES) to estimate real-world methane emissions from LNG-fueled ships using drones, helicopters, and in-stack sensors. Studies like this will more accurately measure methane slip and, if reflected in policies, will help account for and control the climate impacts of LNG-fueled ships. Watch out for that study later this month.

Given that ships can remain in service for decades—the average ship is now more than 22 years old—many of the ships built today will probably still be in the fleet in 2050, when the IMO aims to achieve net-zero emissions. This makes regulations on them crucial, and starting in 2026, the European Union (EU)’s Emissions Trading System (ETS) will cover methane emissions from ships entering or departing EU ports. A separate regulation, FuelEU maritime, will require ships to reduce the life-cycle GHG intensity of on-board energy use starting in 2025. With the FuelEU maritime regulation in effect, ships could only use LPDF 4-stroke engines with 100% fossil LNG if they also use credits from overperforming ships in their fleet or buy credits from other ships; absent that, they will have to use a mix of fossil LNG and qualifying bio- or synthetic fuels. This is because the European Union included methane slip and upstream well-to-tank emissions in the regulations.

The IMO and other multilateral and national authorities could not only follow the EU example but consider more ambitious targets than the European Union has set thus far. IMO delegates are currently developing a GHG Fuel Standard (GFS) to regulate the life-cycle GHG intensity of marine fuels that’s similar to the FuelEU maritime regulation. The earliest the GFS could enter into force is 2027, and if it is to spur emissions reductions that would achieve IMO’s 2050 goal, the GFS will have to break from historical patterns of lagging behind the European Union and be more stringent from the start. After all, the EU regulation aims for 80% reduction in the GHG intensity, not 100%, by 2050. In the meantime, regions or countries could set more ambitious regulations that target methane pollution in their waters. For the more than 150 countries that have signed on to the Global Methane Pledge, reducing methane emissions from ships that call on their ports or sail in their waters would help to achieve the goal of reducing global methane emissions by 30% between 2020 and 2030.

And to be clear, alternatives to LNG are out there. Methanol avoids the methane slip problem and is liquid at room temperature. Other options expected to be available in the longer term include hydrogen fuel cells or batteries. For LNG carriers that continue to use LNG as their fuel, low-methane-slip HPDF engines are a better choice for the environment than the LPDF engines that have long dominated the class. Regulations like a strict GFS that could support the adoption of fuels with lower life-cycle GHG emissions.

In a blog post we’ll publish in a couple of days, my colleagues use our new Polaris model to estimate the life-cycle GHG intensity reductions that would be needed until 2050 to align with IMO goals or with the Paris Agreement. And at the same time, multilateral and national regulators can be ambitious in their own efforts to limit GHG emissions, including methane, from marine fuel.