The Marine Pollution Convention (MARPOL) regulates pollution from shipping. MARPOL Annex VI, which came into effect in 2008, specifically regulates the emissions of sulfur oxides and nitrogen oxides to the air through emission standards with incremental limits over time. High time to check against independently collected data whether the last 15 years of regulations on sulfur oxides and nitrogen oxide emissions from shipping have paid off.

The article hones in on important milestones in the regulations, including the creation of the European Emission Control Areas (ECAs) in the North and Baltic Seas for sulfur dioxide (SO₂) in 2010 and for nitrogen oxides (NO_x) in 2021. In these ECAs more stringent limits were introduced. Within the sulfur emission control area (SECA), the most important reduction was that of the 0.5 to 0.1% fuel sulfur content (FSC) in 2015. Another milestone was the implementation of the global sulfur cap in 2020 which reduced the maximum FSC worldwide with 85% from 3.5 to 0.5%.

To evaluate the impact of these regulatory milestones, we combined and analyzed more than 110,000 remote ship plume measurements and on-board inspection results from 2015-2020. The remote measurements were conducted using either fixed stations in ports or close to passing ships in the Netherlands, Germany and Denmark (fig 1-2). Another way ship exhausts were measured was by using airborne platforms like the Belgian coastguard aircraft (fig 3), but also from Remotely Piloted Airborne Systems (RPAS) (fig 4) and helicopters.

The data demonstrated that compliance rates for sulfur emissions improved drastically since the start of the measurements (fig 5). Furthermore, it was found that in the North Sea coastal states in the period 2015-2022 more than 500 penalties were administered for sulfur infringements. Based on the monitoring and inspection results we concluded that North Sea countries were able to implement an effective compliance monitoring and enforcement system. The same trend was observed throughout the whole European union, in both the Baltic sea and the North Sea.

Our research highlights that the international and European regulations appear to have been very successful in reducing SO₂ emission from ships. A negative side note is that the spatial analysis demonstrates that regulations are better followed near ports than in open waters and non-compliance generally becomes higher closer to the ECA border.  Counterproductively, it was found that the introduction of the global cap limit of 0.5% FSC resulted in a slight increase of the SO₂ emissions in the SECA from 2020 onwards. The reason for this can be traced back to the increased use of exhaust gas cleaning systems (scrubbers). Although these systems are designed to reduce SO₂ emissions, if these systems are not working properly the emissions are in fact much higher than regular ships that are not compliant (fig 6). 

However, for nitrogen oxides it was found that the international ship emission regulations did not affect the real world emissions from ships. It was in fact observed that NO_x emissions from ships even increased. Surprisingly, the measurement data from several monitoring operations indicate that more recently built ships (Tier II) are emitting more NO_x than older ships (Tier I) (fig 7A). This is caused by certain regulatory gaps. One of the problems with the regulations is that they focus on the emissions that are generated at high engine loads, while trends indicate that ships more often sail on lower loads, especially in the ECA and close to shore or port (fig 7B).

 Another problem is that the most recent Tier III ships,  which emit substantially less NO_x, are only slowly being introduced in the merchant fleet. This is due to the fact that the Keel Laying Date (KLD) is used in the regulations for the determination of the tier level. As the registration of future built ships can be done many years in advance, ships can therefore have a KLD prior to the construction. The data demonstrates that the majority of the newly built ships and the ships planned for the next following years have a KLD dating before 2021 (fig. 8A), which means they are still subject to the less stringent Tier II emission levels.

Furthermore, for the very low number of measured Tier III ships, very high potential non-compliance levels were observed (fig 8B). This is not only reflected in the remote measurement data,  satellite data also indicate that the NO_x emissions at sea are increasing instead of decreasing (fig 9).

Although the data clearly demonstrates that there are a non-negligible amount of non-compliant ships in the ECA, only 1 NO_x penalty was imposed in the Bonn Agreement area. The difference in administered fines for SO_x infringements compared to NO_x ones is the result of the lack of EU regulations on NO_x and the lack of an effective NO_x enforcement mechanism for on-board inspections in ports.

The findings of our research are especially relevant for policy makers and other stakeholders responsible for environmental regulation and enforcement, as they can offer valuable guidance in developing more efficient regulations and strategies for monitoring and enforcing of compliance od emission regulations at sea and in port.

https://rdcu.be/dpwCA

https://doi.org/10.1038/s43247-023-01050-7