Regulate short-lived pollutants in maritime transport

Black carbon (BC) and methane (CH4) are known as ‘short-lived’ climate pollutants because they last less time in the atmosphere compared with CO2. However, on a pound-for-pound basis, they have a far more powerful climate-warming effect than CO2­. CH4 is 30 times more powerful than CO2 for every kilogram emitted based on its 100-year global warming potential (GWP100). The effects of BC are even greater, though they are dependent on the location of the emissions and atmospheric feedback effects (IPCC, 2021). BC emissions are particularly high in the Arctic as it changes the reflectivity of Arctic ice making it absorb more sunlight and accelerate melting (Bond et al., 2013). BC is also a local air pollutant with negative health effects.

The latest International Maritime Organisation (IMO) greenhouse gas emissions study estimated that CH4 and BC accounted for 0.48% and 6.85% of voyage-based international carbon dioxide equivalent emissions in 2018. CO2 and nitrous oxide (N2O) emissions accounted for the remaining 91.32% and 1.35%, respectively (IMO, 2020a), all based on GWP100. However, short-lived climate pollutant emissions from shipping are increasing with growing demand; CH4 emissions have grown by 151-155% over the period 2012-18, primarily linked with the increase in the liquefied natural gas (LNG) carrier fleet; BC emissions grew by 11.6% over the same period. Reductions in short-lived climate pollutants are important to limit the effects of climate change.

BC emissions are a component of particulate matter produced during the combustion process in an engine. BC emissions are particularly high for engines using the heavy fuel oil that dominate shipping fuel use. BC emissions can be effectively reduced using exhaust gas treatment technologies such as particulate filters. Additional improvements can also be attained with improvements in engine tuning, though the effects of SOx scrubbers are uncertain (Aakko-Saksa, 2018). The vast majority of ships are not currently fitted with these technologies because BC remains unregulated, however. BC and other particulate matter could also be reduced significantly by switching from heavy fuel oil to distillate fuels, LNG, methanol or alternative renewable fuels like hydrogen and ammonia (ICCT, 2019). LNG emits methane, methanol emits carbon dioxide, and ammonia emits nitrous oxide, however.

Using LNG in ships reduces BC and other pollutants such as sulphur oxides, NOX and particulate matter. However, its use often leads to CH4 emissions from ‘methane slip’, where a fraction of natural gas passes through the engine unburnt and are released into the atmosphere. The severity of methane slip is highly dependent on the type of engine design used; methane emissions from high-pressure dual fuel (HPDF) engines are an order of magnitude lower than low-pressure dual fuel (LPDF) engines. Currently, most new LNG ships are fitted with poorly performing LPDF engine technology (Pavlenko et al., 2020) because they are less expensive than HPDF engines which require costly exhaust gas treatment technologies to comply with NOx regulations. Methane emissions also occur from the extraction, processing and distribution of fossil gas to power LNG ships (ITF, 2020).

Several emissions control areas (ECAs) exist in North America and European waters, limiting the emissions of local air pollutants and thereby indirectly incentivising reductions in black carbon emissions because many ships comply by switching from residual fuels to distillates. Local policies explicitly targeting particulate matter emissions are also in place in four port cities in North America - Los Angeles, Long Beach, Oakland and Vancouver - which were estimated to have reduced black carbon emissions by 81% between 2005 and 2013 using incentives (Brewer, 2019).

In June 2021, IMO MEPC 76 adopted amendments to prohibit “the use and carriage for use as fuel of heavy fuel oil (HFO) by ships in Arctic waters on and after 1 July 2024”, which would indirectly help to reduce BC emissions in the Arctic. However, a number of exemptions would reduce the effectiveness of the regulation; ships flying flags from Arctic states or with protected fuel tanks will be able to apply for a waiver until 2029 (IMO, 2021). Explicit measures targeting BC emissions such as ECAs in the Arctic are yet to be formally discussed at IMO.   

Existing IMO policy instruments on climate change, such as the Energy Efficiency Design Index, and emissions trading schemes like the EU ETS target CO2 emissions, mean that other greenhouse gases and climate pollutants such as CH4 and BC are omitted from policy coverage (ITF, 2020). No global regulations limit CH4 or BC emissions in maritime shipping, meaning shipping companies have few incentives to adopt higher performing engine technologies, after-treatment devices, or switch to lower emission fuels since all entail additional costs. Introducing regulations defining CH4 and BC emissions limits as part of a ship’s emissions certification process could address this. Similar regulations already exist at the IMO on air pollutant emissions such as NOx and SOx, and shipping companies can ensure compliance by either adopting engine and after-treatment technologies or by using lower-emission fuels.

While there are many ways to measure methane and black carbon, there are not yet internationally agreed standardised testing procedures in place for CH4 and BC, meaning new regulatory developments are needed. The IMO Marine Environment Protection Committee (MEPC) is due to consider how to develop standardised measurement procedures for BC, possibly by adapting NOx certification procedures, and investigate how they can be used to develop policy options (ICCT, 2019; IMO, 2020b). Testing procedures must be representative of real-world vessel operation and account for the range of influencing factors including fuel specification, engine type and operational and maintenance conditions. Regulations on short-lived climate pollutants could be highly effective if their scope is global. A regional scope - particularly for black carbon in the Arctic – would also be possible and would require fewer negotiating parties.