Maritime shipping accounts for nearly 3% of global CO2 emissions, with heavy fuel oil (HFO) and scrubber systems dominating propulsion. Hydrogen emerges as a potential decarbonization pathway, particularly for short- to medium-range vessels. A well-to-wake analysis reveals significant differences in emissions, water use, and ecological impacts between hydrogen-based propulsion and conventional marine fuels.

**Lifecycle Emissions: Hydrogen vs. Conventional Fuels**
Hydrogen combustion in marine engines produces zero CO2 emissions at the point of use. However, upstream emissions depend on production methods. When hydrogen is derived from renewable-powered electrolysis, well-to-wake CO2 emissions range between 0.5 and 2 kg CO2 per kg H2, primarily from equipment manufacturing and transport. In contrast, HFO combustion emits approximately 85 kg CO2 per GJ, with scrubber systems adding 5-10% energy penalties due to backpressure and sludge disposal. Scrubbers also convert SOx emissions into sulfate discharges, contributing to ocean acidification.

Methane slip is a critical concern for liquefied natural gas (LNG)-fueled ships, with slip rates between 0.2% and 6% of total fuel consumption. Methane’s 28-34x global warming potential over 100 years exacerbates climate impacts. Hydrogen systems eliminate methane slip but face challenges with NOx formation during combustion. Advanced fuel cells or catalytic burners can limit NOx to <10 ppm, comparable to Tier III marine engine standards.

**Water Use in Marine Hydrogen Systems**
Water consumption for onboard hydrogen production via electrolysis is approximately 9 liters per kg H2. This is negligible compared to the 1,000-3,000 liters of ballast water discharged daily by mid-sized cargo ships. Open-loop scrubber systems, meanwhile, consume 30-50 m3/MWh of seawater, releasing acidic effluent with heavy metals. Closed-loop scrubbers reduce water intake but require chemical additives like NaOH, creating hazardous waste streams.

**Ecological Benefits and Trade-offs**
Hydrogen propulsion eliminates particulate matter (PM2.5) emissions, a major health hazard near ports. HFO combustion emits 1-3 g/kWh of PM, while scrubbers reduce this by 60-80% but increase nano-particle emissions. Hydrogen also avoids black carbon, which accelerates Arctic ice melt. However, LH2 storage requires cryogenic tanks, increasing vessel weight and reducing cargo capacity by 5-15%. Ammonia as a hydrogen carrier offers higher energy density but introduces toxicity risks in spills.

**Operational Comparisons**
Energy efficiency favors hydrogen in smaller vessels. Fuel cell systems achieve 40-50% efficiency, versus 35-45% for modern diesel engines. For large container ships, hydrogen’s low volumetric energy density (8 MJ/L for LH2 vs. 36 MJ/L for HFO) limits range without frequent bunkering. Synthetic e-fuels like methanol, derived from green hydrogen, may bridge this gap but at higher well-to-wake emissions (20-30 kg CO2/GJ).

**Regulatory and Infrastructure Gaps**
IMO’s Carbon Intensity Indicator (CII) regulations penalize HFO use, but hydrogen lacks standardized bunkering protocols. Pilot projects show LH2 bunkering takes 2-3x longer than HFO refueling. Ports must invest in cryogenic handling equipment, with costs 5-10x higher than conventional fuel terminals. Safety protocols for hydrogen dispersion in confined docks remain under development.

**Future Trajectories**
Short-sea shipping and ferries are early adopters due to lower range requirements. The EU’s FuelEU Maritime initiative incentivizes hydrogen use, targeting 2% renewable fuel uptake by 2030. Retrofitting existing ships is costly (20-30% of newbuild price), making newbuilds the primary pathway. Hybrid systems combining hydrogen with batteries may optimize efficiency for auxiliary power.

The transition hinges on scaling renewable hydrogen production and standardizing marine-grade fuel cells. While hydrogen addresses emissions and local pollution, its viability depends on overcoming energy density limitations and infrastructure barriers in global shipping routes.