The shipping industry’s nitrogen pollution problem

23 Apr

Pollution from the shipping industry takes many forms: CO2 released from engines, plumes of black particulates billowing from exhausts, and wastewater poured overboard from scrubbers. Nitrogen, however, often slips under the radar. But what is shipping’s nitrogen pollution problem, and how can the International Maritime Organization and governments take action?

At the 83rd meeting of the Marine Environment Protection Committee (MEPC 83) this month, the International Maritime Organization (IMO) agreed measures to cut the shipping industry's greenhouse gas (GHG) emissions. The agreement puts a global price on shipping’s GHG emissions, but falls way short of the most ambitious proposals championed by climate vulnerable countries.

It was this decision that dominated most of the attention garnered by MEPC 83, but in a separate room at the IMO, member states made another significant move to approve the world's largest Emission Control Area (ECA) in the North-East Atlantic Ocean. The ECA places limits on harmful pollutants across swathes of the North-East Atlantic, with anticipated benefits for public health and the environment. The pollutants in question include nitrogen oxides (NOx), one form of damaging nitrogen pollution resulting from shipping. So why is nitrogen a pollutant, and how much of it comes from shipping?

Humans have dangerously disrupted Earth’s nitrogen cycle

Nitrogen gas makes up nearly 80% of our atmosphere, where it exists as a stable, unreactive molecule made of two nitrogen atoms tied together by a strong chemical bond. However, if that bond is broken, reactive nitrogen compounds like ammonia (NH3), nitrous oxide (N2O) and nitrogen oxides (known as NOx) can be formed. In the environment, these compounds pass through a chain of reactions before they eventually form nitrogen gas once more: this is known as the global nitrogen cycle.

Flows of nitrogen, together with phosphorous, are essential for life and are one of nine key processes responsible for the stability of our planet. However, since the 20th century, the use of fertilisers for agriculture and burning of fossil fuels have massively disrupted the global nitrogen cycle. As a result, the cycle has been pushed beyond safe planetary boundaries, heightening the risk of irreversible Earth system change.

*The* *planetary boundaries framework* identifies nine key processes which control the stability of our planet, one of which is biogeochemical flows of nitrogen and phosphorous. As this figure illustrates, the use of nitrogen-based fertiliser and burning fossil fuels has dangerously disrupted the nitrogen cycle beyond the “safe operating space”, increasing the risk of rapid and irreversible Earth system change. Credit: Azote for Stockholm Resilience Centre, based on analysis in *Richardson et al. (2023). Licensed under* *CC BY-NC-ND 3.0*.

Whilst the agriculture sector is often (correctly) highlighted as the major contributor to nitrogen pollution worldwide, the shipping industry also plays a significant role. And, because of the location of shipping activities, this pollution can have unique effects on our health, the environment and climate.

Nitrogen pollution from the shipping industry

The vast majority of ships operating today are powered by fossil fuels. While these fuels are carbon-based, nitrogen pollutants like N2O and NOx can be formed as waste products in ship engines, and emitted to the atmosphere. This nitrogen pollution causes significant challenges.

1. Nitrogen pollution reduces air quality

NOx are important contributors to poor air quality. While NOx emissions come from many sources, shipping is one of the most important: in the UK, domestic shipping contributes 11% of NOx emissions. Meanwhile, international shipping in UK waters (which isn’t counted in the UK’s domestic NOx emissions total) emitted almost three times more NOx than UK domestic shipping in 2023.

Worldwide, air pollution from the shipping industry contributes to as many as tens or even hundreds of thousands of additional premature deaths every year. These health impacts from shipping’s air pollution are disproportionately shouldered by communities close to ports and major shipping lanes.

2. Nitrogen pollution harms the environment and biodiversity

Nitrogen is a key ingredient for life, but in excess its reactive form can cause serious environmental damage. Too much reactive nitrogen causes eutrophication, which can lead to the rapid growth of plants and algae in aquatic and marine environments. This can cause oxygen levels in the water to plummet, with potentially deadly consequences for marine species and ecosystems, including those that coastal communities around the world rely on as their primary source of food.

3. Contributing to climate change

N2O is a powerful greenhouse gas 273 times more potent than CO2. N2O emitted by ships today accounts for only about 1.5% of shipping’s overall GHG emissions, the majority of which are CO2.

However, nitrogen pollution also contributes to climate change in other ways. For instance, reactive nitrogen can damage and degrade important “blue carbon” habitats that sequester and store significant amounts of carbon. Damage to these natural carbon sinks, which include mangroves and seagrass meadows, can both reduce their capacity to absorb carbon, and cause the release of carbon stored in soils and sediments. Additionally, when released into the environment, NOx and NH3 (forms of reactive nitrogen which are not GHGs themselves) can be converted to N2O by chemical reactions as part of the nitrogen cycle. The resulting GHG emissions are referred to as indirect N2O emissions.

Nitrogen and shipping decarbonisation

Nitrogen pollution poses a problem for some prospective pathways to cut shipping emissions currently making headway. Most of shipping’s GHG emissions result from fossil fuel combustion, making alternative, low-emission fuels crucial.

One candidate is ammonia (NH3), a form of reactive nitrogen which doesn’t produce CO2 when burned. However, the potential benefits of switching fuels to NH3 are complicated by nitrogen pollution. The environmental, climate and public health risks outlined above may be ramped up by the uptake of NH3 as a fuel – and not just when used onboard but across its lifecycle:

NH3 fuels carry the risk of releasing NOx and unburned NH3 emissions, which are both harmful contributors to poor air quality. Without new controls on these emissions, switching the entire global shipping fleet to NH3 could result in more than 600,000 additional deaths every year.
More broadly, NH3 leaks, spills and emissions will be inevitable between the process of NH3 production and final use onboard ships. As well as heightening overall risks to climate, the environment and public health, NH3 is extremely toxic, creating severe environmental risks and safety concerns for supply chain and port workers, as well as mariners.
Burning NH3 on ships can produce more N2O than fossil fuel-powered engines. Increased leaks, spills and emissions of other reactive nitrogen compounds (like NH3 and NOx) could also lead to increased indirect N2O emissions. Currently, the actual amount of N2O emissions released across the NH3 lifecycle is uncertain, with a wide range of values recorded. At the high end of this range, N2O emissions significantly offset the GHG emissions savings compared with using fossil fuels.

NH3 (when produced using green hydrogen) has the potential to meaningfully reduce GHG emissions for an industry in urgent need of decarbonisation. Making this a reality, however, will require effective technology, policy and regulation. These processes must also keep a keen eye on nitrogen pollution or else merely substitute one set of pollutants for another, at the expense of the environment, climate, and public health.

Current efforts to control nitrogen pollution

To date, efforts to tackle nitrogen pollution from the shipping industry have focused on reducing NOx emissions. Key to these efforts are ECAs, which apply legal limits on the emission of air pollutants, including NOx, from ship engines. The decision to approve the North-East Atlantic ECA at MEPC 83 was undoubtedly a positive step: research from the International Council on Clean Transportation estimates the new ECA could prevent up to 4,300 premature deaths between 2030 and 2050.

However, it will take time before we see these NOx emissions fall, not least because the strictest limits only apply to ships built on or after the ECA implementation date. NOx emissions also remain unregulated across large parts of the world’s oceans and seas, making further ECA expansion a priority. Beyond this, ECAs are an effective solution to fossil-fuelled ships, but a fresh regulatory framework will be needed to limit N2O and unburned NH3 emissions from NH3 as shipping sails into a new era of alternative fuels.

Immediate action is needed

We need urgent action to curb shipping’s nitrogen pollution problem now and provide safeguards for the future. The UK House of Lords Environment and Climate Change Committee’s nitrogen inquiry marks a step towards a clearer view of and bolder steps towards confronting this problem, but we must go further. The IMO and UK government must exert tighter controls on nitrogen pollution from shipping with the following three steps:

1. Strengthen efforts to improve air quality

The decision to adopt the North-East Atlantic ECA was an important step, but further expanding ECAs is essential to reduce NOx emissions. In the future, expanding ECAs and including specific limits on NH3 emissions could help reduce the public health impacts of ships operating using NH3.

2. Monitor and control N2O emissions from NH3 ships

Few ships currently use NH3 fuel. As industry frontrunners emerge, they must monitor N2O emissions using Continuous Emissions Monitoring techniques, and report the results publicly.
Policies must incentivise reducing N2O emissions, and ensure that shipping companies pay their fair share for N2O emissions. Default N2O emissions factors for NH3, used by shipping companies to calculate their emissions for Emissions Trading Schemes (ETS), should be set to high values.

3. Monitor and tightly control NH3 leaks, spills and emissions across the fuel lifecycle

There should be requirements for transparent reporting of NH3 leaks, spills and emissions from the supply chain and onboard vessels.

The shipping industry has had a nitrogen pollution problem for decades, and the far-reaching effects have long been felt by communities and ecosystems across the world. While the shipping industry is right to explore alternative fuels including NH3, we need to take every precaution to ensure it does not exacerbate these impacts – or even cancel out any positive climate effects. Technology, policy and regulation are all part of the answer, and the time to act is now.

Learn more in our written and oral submissions to the UK House of Lords Environment and Climate Change Committee’s nitrogen inquiry, and additional information we provided upon request with the SASHA Coalition.