Ammonia in the Spotlight of Maritime Decarbonization

As the shipping industry grapples with its colossal carbon footprint, responsible for nearly 3% of global greenhouse gas emissions, ammonia has emerged as a hopeful alternative to conventional fuel oils. The International Maritime Organization has introduced a stringent global fuel standard set to be adopted in October 2025. This standard assesses fuels on their full lifecycle emissions, from production to combustion. Ammonia, with no carbon dioxide (CO₂) emissions at the point of use, is under intense scrutiny as a potential climate-friendly shipping fuel.

However, recent evidence from scientific research and industry collaboration, including a lifecycle study led by Maersk and a climate impact model developed by the Environmental Defense Fund, reveals a more complex and cautionary narrative. Ammonia’s indirect emissions,especially reactive nitrogen compounds like nitrous oxide (N₂O), nitrogen oxides (NOₓ), and unburned ammonia,  may significantly undercut its perceived climate benefits.

Divergent Pathways of Ammonia Production

The climate impact of ammonia as a marine fuel begins with how it is produced. The traditional "gray" ammonia is made using hydrogen derived from natural gas or coal, releasing vast amounts of CO₂ and methane. This production method alone contributes to over 1.2% of global GHG emissions, equivalent to the annual emissions of about 150 million cars.

In contrast, alternative production methods such as "blue" ammonia and "e-ammonia" are designed to reduce emissions. Blue ammonia incorporates carbon capture and storage, whereas e-ammonia uses renewable energy for hydrogen production through electrolysis. E-ammonia, especially when powered entirely by wind or solar energy and paired with leak-free hydrogen systems, has shown up to 80% lower lifecycle GHG emissions compared to very low sulfur fuel oil (VLSFO). Blue ammonia, depending on CCS efficiency and methane leakage control, achieves about 60% emissions reduction.

Combustion Complications: The Rise of Reactive Nitrogen

Burning ammonia in ship engines may avoid CO₂ emissions, but it introduces a new suite of pollutants. Ammonia combustion emits reactive nitrogen compounds such as N₂O, NOₓ, and traces of unburned ammonia. N₂O, in particular, is 273 times more potent than CO₂ and remains in the atmosphere for more than 120 years. The unintended release of these gases raises major environmental concerns, as they contribute to global warming, air pollution, and ecosystem degradation.

Notably, the Life Cycle Assessment study by Maersk and Danish consultancy LCA 2.0 incorporated these reactive nitrogen emissions, setting a new benchmark for evaluating marine fuels. Their analysis found that while e-ammonia can meet the IMO’s near-zero threshold (≤19.0 g CO₂e/MJ before 2034), its performance is highly sensitive to even small leaks or combustion inefficiencies.

The Silent Spread of Indirect N₂O Emissions

Ammonia’s climate risks are not limited to ship engines. Across the fuel’s value chain, during production, storage, transport, and bunkering, ammonia can leak, evaporate, or be vented, especially in gaseous or liquefied states. Once in the environment, it undergoes biological and chemical transformations that can lead to indirect N₂O emissions. Agricultural studies have shown that 0.1% to 21% of released ammonia-nitrogen may convert into N₂O, depending on environmental factors such as soil moisture, oxygen levels, and microbial activity.

Yet, these indirect emissions are frequently omitted in lifecycle assessments. The LCA study included them in a secondary sensitivity analysis using a 0.3% conversion rate. EDF, concerned that this figure might be too low, modeled scenarios with higher conversion rates to better understand potential worst-case outcomes.

EDF’s Comprehensive Climate Model for Ammonia Fuels

To close the data gap, the Environmental Defense Fund developed a value chain model that simulates N₂O, NOₓ, and ammonia emissions from each phase of the ammonia marine fuel lifecycle. The study, Climate Impact of Direct and Indirect N₂O Emissions from the Ammonia Marine Fuel Value Chain, published in Environmental Science & Technology, maps out scenarios from best-case (low emissions, low conversion) to worst-case (high emissions, high conversion rates).

Under low-emission scenarios, e-ammonia could still cut emissions by 70–80% compared to fuel oils. However, if nitrogen-to-indirect-N₂O conversion exceeds 5%, ammonia could lose all its climate advantage—and in extreme cases, become more polluting than current marine fuels. For blue ammonia, the margin is even tighter. It can only deliver around 30% reduction in lifecycle GHG emissions and may perform worse than VLSFO under high nitrogen emission conditions.

Environmental Trade-Offs Beyond Climate Impact

The LCA study also highlighted non-climate concerns. When compared to VLSFO, ammonia use increases the potential for negative impacts on terrestrial and aquatic ecosystems by 35–65%. This is primarily due to ammonia's toxicity and its role in forming particulate matter when it reacts with sulfur and nitrogen oxides in the atmosphere.

These ecological trade-offs demand the installation of mitigation systems onboard ships. Technologies such as Selective Catalytic Reduction (SCR) units and ammonia slip catalysts are essential to control NOₓ and unburned ammonia, respectively. Furthermore, investments in double-walled containment and active leak detection systems are crucial for minimizing spills during transport and bunkering.

Policy Direction: Tightening the Imo’s Regulatory Net

The IMO’s October 2025 deadline for fuel regulation adoption is a pivotal moment. Experts and environmental advocates argue that its standards must be rigorous and comprehensive. All GHGs, CO₂, CH₄, H₂, N₂O, as well as reactive nitrogen species like NH₃ and NOₓ, should be included in the assessment framework. Fuel certification systems must require third-party validation and lifecycle carbon intensity disclosure.

In parallel, governments and port authorities must enforce fuel-handling regulations, incentivize clean production methods, and fund further research into emission control technologies. Without these safeguards, the shipping sector risks trading one environmental liability (carbon) for another (nitrogen).

The Path Forward: Vigilance, Transparency & Innovation

Ammonia’s potential as a marine fuel lies in a delicate balance. While it offers significant promise for deep decarbonization, especially as e-ammonia, the complexity of its emissions calls for a meticulous and vigilant approach. Industry collaboration, rigorous modeling, and policy foresight will be necessary to ensure ammonia becomes a truly climate-aligned alternative rather than a Trojan horse for unintended environmental harm.

Key Takeaways:

• Ammonia as a marine fuel emits no CO₂ during combustion but can release potent GHGs like N₂O and other nitrogen pollutants.

• E-ammonia can achieve up to 80% lifecycle GHG reductions if reactive nitrogen emissions are minimized; blue ammonia’s benefits are far more limited.

• Indirect emissions across ammonia’s value chain, such as leaks and transformations into N₂O, can negate its climate benefits unless strictly controlled.