Why Understanding CO2, NOx, and SOx Emissions in Shipping and Ports is important

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How can the maritime sector, through embracing a collaborative approach, unlock synergies and efficiencies that benefit shipping and ports while advancing toward shared sustainability goals? It will require collaboration between all parties to break down the silos, fostering open communications and cooperation and viewing emissions reduction as a collective endeavour rather than an individual mandate. Ultimately, by working together, shipping and ports  (good example of this is the Singapore Green Initiative) can maximize their positive impact on the environment, society and the economy, paving the way for a more sustainable future.

This is why understanding CO2, NOx, and SOx emissions in shipping and ports is crucial for addressing maritime pollution’s environmental and health impact. 

Putting this into context, while the maritime sector serves as the backbone of global trade and will continue to do so, it also contributes significantly to emissions of carbon dioxide (CO2), nitrogen oxides (NOx), sulfur oxides (SOx), and particulate matter (PM). These emissions mainly result from fossil fuel combustion and have massive implications for climate change and importantly, port air quality.

Global and regional regulations are also driving this issue. The International Maritime Organization (IMO) has set targets to reduce greenhouse gas (GHG) emissions from international shipping, with a goal of achieving net zero CO2 emissions by 2050. There is also the Paris Agreement and the European Union’s Green Deal, as well as pressure from local surrounding communities, NGOs and other stakeholders.

Read the article: Maritime Decarbonization: Ports and Green Shipping Regulations in Europe

Is it unfair that ports are pursuing decarbonisation without unified regulations?

Ports are looking to decarbonize their operations but don’t have the same global regulator driving them. However, they have recognized the need to reduce emissions and identified the prime sources as their port vehicles and employees travelling to and from, ship calls and berthing, land transport (HGVs, etc.), and storage/warehousing facilities. 

GHG Rating allows ports to assess the emission profile of any vessel in their port, enabling them to develop effective decarbonization strategies such as port incentive programmes to attract more efficient vessels.

While CO2 is the primary contributor to maritime sectors’ climate impact, it’s essential to address all three main sources of emissions: CO2, NOx, and SOx. 

Current regulations targeting SOx and NOx are focused on ships. They are being tackled through shipping fuels and slow steaming, with ongoing efforts to develop methodologies like well-to-wake (WTW) and lifecycle assessment (LCA) to comprehensively assess the climate impact of fuels.

How do you define shipping decarbonization?


  • Shipping decarbonization can be defined as the process of eliminating ships’ CO2, NOx, SOx and other emissions. Through mitigation measures leading to net zero COs emissions by 2050. The IMO has pleaded to reduce the total GHG emissions from international shipping by at least 20% by 2030 and at least 70% by 2040, with a goal of net zero emissions by 2050.
  • Port decarbonization is technical, but the aim is for operational emissions to be reduced, eradicated, and offset from its sources (operations, ships, land, transport, storage & warehousing) and be net zero by 2050.

However, the challenge extends beyond individual vessels or port operations; it encompasses the entire supply chain and value network. This integrated approach acknowledges that emissions generated by ships don’t end at the port’s edge but continue throughout the cargo handling process and beyond.

Read the article: Understanding Scope 3 Emissions: The Potential of Port Sustainability

While carbon dioxide (CO2) is the main source of shipping’s climate impact, it is essential to look at the main three sources:

Why we need to understand the impact of Carbon Dioxide (CO2)

The combustion of fossil fuels in main engines, auxiliary engines, boilers, and incinerators onboard ships is the primary source of CO2 emissions. These emissions directly threaten marine ecosystems as they are absorbed by seawater, impacting the oceans’ ability to produce up to 60% of the oxygen we breathe.

Mitigating CO2 emissions is, therefore, crucial for combating climate change and preserving marine biodiversity. According to a report by The World Economic Forum in 2018, if shipping were a country, it would rank as the sixth biggest carbon polluter, underscoring the urgency of addressing CO2 emissions in the maritime industry.

Highlighting the significance of GHG Rating, our CEO, Sjoerd de Jager, emphasizes its role in enabling ports to assess the emission profile of any vessel in their port. This empowers ports to develop effective decarbonization strategies, including incentive programmes to attract more efficient vessels.

 Emissions in Shipping Sjoerd de Jager PortXchange
Image ©Licensed to Ben Stevens Photography
Waterfront Conference Company UK Ports Conference 2024

While regulations primarily target shipping lines, Sjoerd underscores ports’ vital role in meeting these challenges. He asserts that ports are strategically positioned to lead the charge towards greener shipping, given their essential role in designing new fuel infrastructure and their status as sources of significant emissions from transportation and industrial cluster activity.

Importantly, he notes that port emissions cannot be overlooked in shipping line calculations, as they often fall under Scope 3 and sometimes Scope 2 emissions. Given that ports are frequently situated near residential areas, the impact of a port’s decarbonization agenda is profound and far-reaching for their local communities and countries.

Understanding of the environmental impact of derived from shipping vessels’ fuel combustion    

Shipping Tank-to-Wake are the emissions resulting from fuel use onboard a marine vessel. This includes propulsion, energy generation, and auxiliary systems. Many sources have suggested vessel speed reduction as an effective way to reduce CO2 emissions from global shipping. This is especially true for container ships, which, despite making up only 4% of the global shipping fleet, contribute 22% of CO2 emissions. Speed reductions of 10%, 20% and 30% reduced CO2 emissions by 19%, 36% and 51%, respectively. 

For port cities in particular, vessel speed reduction can positively affect air quality due to reducing the amount of fuel being used.

Should the Shipping Industry Face Stricter Sulphur Limits to Combat Acid Rain?

According to analytics from the European Geosciences Union, shipping is responsible for approximately 10% of the sulfur oxides (SOx) emitted globally. When sulfur oxides react with nitrogen dioxide (NO2) in the atmosphere, they form sulfuric acid, a key component of acid rain.

Additionally, SOx emissions contribute to the formation of harmful secondary inorganic aerosol gases, posing risks to human health and ecosystems. The quantity of SOx emissions is directly correlated with the sulfur content of the fuel used in ships, highlighting the importance of transitioning to cleaner fuel alternatives.

The IMO has implemented its regulations limiting the sulphur content of marine fuels for ships as specified in Regulation 14 (Sulphur Oxides (SOx) and Particulate Matter (PM)) of Annex VI of MARPOL 73/78, in which the sulphur content limit has been gradually strengthened. 

Since the limit for open sea area was reduced from 3.50% to 0.50% on 1 January 2020,  the use of marine fuel oil with a sulphur content complying with the regulations (compliant fuel) and the installation of SOx scrubber or converting to alternative fuels such as LNG has been adopted. 

Also, a more strict sulphur content limit is applied in Emission Control Areas (ECA) than in the open sea area. ECAs are sea areas that limit SOx and NOx emissions by 0.10% [m/m] of fuel for SOx and Tier III for NOx (pending vessel age). IMO, Regulations under MARPOL Annex VI state all ECAs are SOx-constrained (SECAs).

SOx sanctions within ECAs are established by the individual parties who have signed up to MARPOL, such as flag and port states. There is no established fine or sanction set by IMO – it is down to the individual State Party.

How Can Shipping Reduce NOx Emissions?

Nitrogen oxides (NOx) are toxic gases formed by the high-temperature combustion of fuels, including oil, diesel and gas. These emissions contribute to air pollution and harm human health, ecosystems and the climate. 

Ship emissions contribute approximately 15% to global NOx emissions, as estimated by EGU analytics. NOx acts as a catalyst for acid rain and ground-level ozone formation, exacerbating respiratory problems and harming plant and animal life. Addressing NOx emissions in shipping requires adopting technologies and practices that minimize combustion temperatures and optimize fuel efficiency.

IMO Regulations under MARPOL Annex VI state that at the time of writing all ECAs are NOx constrained (NECAs). However, the application of NOx Tier II in the ECAs applies when the ship is constructed on:  

  • 1 January 2016 and operating in the North American ECA and the United States Caribbean Sea ECA; or
  • 1 January 2021 and operating in the Baltic or North Sea ECA. 
  • Any pre-2000 vessel has to comply with the Tier I NOx limit.  

2008 Annex VI  Amendments (Tier II/III)  adopted in October 2008 introduced new fuel quality requirements beginning from July 2010, Tier II and III NOx emission standards for new engines and Tier I NOx requirements for existing pre-2000 engines. 

Furthermore, it applies retroactively to new engines greater than 130 kW installed on vessels constructed on or after January 1, 2000, or which undergo a major conversion after that date. The regulation applies to fixed and floating rigs and drilling platforms (except for emissions associated directly with the exploration or handling of sea-bed minerals).

NOx Sanctions within ECAs are established by the individual parties who have signed up to MARPOL, such as flag and port states. There is no established fine or sanction set by IMO – it is down to the individual State Party.

Addressing Emissions in Shipping

Efforts to reduce CO2, NOx, and SOx emissions in shipping are multifaceted and require collaboration among stakeholders, including shipowners, operators, regulators and international organizations. 

By embracing technological innovations, real-time data impact analysis and the Importance of Port Call Optimisation on Carbon Emissions through JIT (Just in Time) port calls, shipping and maritime are now able to advance their efforts in decarbonizing and reducing the impact they have on people and the planet. 

Digital technologies are crucial in driving this transition, placing sustainability at the forefront of corporate agendas. As vital global supply chain nodes, Ports have a unique opportunity to lead the change towards a greener economy. 

See how the Port of Rotterdam is driving change and reducing their impact on climate change.


EmissionInsider helps ports visualize emissions from all 4 modalities – sea-going vessels, barges, trucks, and rail. Equipped with this data, they can pinpoint the emission sources and develop a targeted approach to lower them. Ships are the single largest source of transport-related port pollution; that’s why EmissionInsider provides granular reporting on emissions per vessel, type, facility, etc.

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It also gives insight into vessel emission levels when berthed at the quay, information that is useful in developing shore power projects. The solution can clarify how much air pollution shore power connections prevent. Additionally, EmissionInsider calculates voyage emissions from upstream and downstream ports.

By leveraging science-based solutions like those offered by PortXchange EmissionsInsider, ports can enhance their efficiency, transparency, and profitability while reducing their environmental footprint. This collaboration represents an important moment in our collective efforts to create a more sustainable and prosperous future.

If you want to see how you can make a real difference, contact us today for a free demo. 

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About the author

Sue Terpilowski

OBE: Shaping Maritime Excellence with Over 36 Years of Industry Insight

Stefana Sopco

Marketing Manager

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