Shipping CO2 emissions are the greenhouse gases released by the global merchant fleet, from the massive container vessels that carry consumer goods to the tankers hauling crude oil and the bulk carriers loaded with grain or iron ore. The industry accounts for roughly 2–3% of global CO2 emissions, a share comparable to a mid-sized industrialized nation, and that share has been climbing. Research tracking the global fleet found a 1.7-fold increase in shipping CO2 emissions over recent decades, driven in large part by the explosive growth of containerized trade.1One Earth. What Are Shipping CO2 Emissions and How Can We Reduce Them? Reducing those emissions is not a single technical fix but a tangle of fuel chemistry, ship design, fleet economics, and international regulation, each with its own timeline and trade-offs.
How the Emissions Have Shifted
The headline number, a nearly two-fold rise in total shipping CO2, hides a structural change underneath. Container ships went from contributing about 14% of global shipping emissions to roughly 40%.1One Earth. What Are Shipping CO2 Emissions and How Can We Reduce Them? That mirrors the broader economic story: the world now moves far more manufactured goods in standardized containers than it did a generation ago. Tankers and bulk carriers still account for a large chunk, but the balance has tilted. Understanding which ship types are responsible matters because each has different operational profiles, route patterns, and realistic decarbonization options. A slow-moving bulk carrier loaded with coal faces different constraints than a container ship running on a tight schedule between Shanghai and Rotterdam.
Counting Emissions the Right Way
Most official emission inventories measure only what comes out of the ship’s funnel, a metric known as “tank-to-wake.” But producing and transporting the fuel to the vessel also generates CO2. When researchers account for those upstream emissions using a “well-to-wake” framework, the numbers jump. One detailed bottom-up model estimated that including fuel production and processing adds about 11% to CO2 totals for the current fleet.2PubMed Central. Global Shipping Emissions from a Well-to-Wake Perspective: The MariTEAM Model That gap will grow as shipping moves toward synthetic or electro-fuels whose manufacturing is energy-intensive. E-fuels, produced using renewable electricity to synthesize hydrocarbons or ammonia, could double or even triple the sector’s total energy consumption on a well-to-wake basis.3Transportation Research Part D: Transport and Environment. Reduction of maritime GHG emissions and the potential role of E-fuels
This has practical consequences. A fuel that looks clean at the smokestack might still carry a heavy carbon footprint if it took enormous amounts of fossil energy to produce. Regulators and industry groups are slowly shifting toward well-to-wake accounting, but the transition is uneven. Any comparison of alternative fuels that only looks at what the engine emits is telling you less than half the story.
What the Regulators Are Demanding
The International Maritime Organization (IMO) set out a revised greenhouse gas strategy in 2023. The targets call for cutting annual international shipping emissions by at least 20% by 2030, at least 70% by 2040, and reaching net-zero by or around 2050, all compared to 2008 levels. The strategy also aims for at least 5% of total shipping energy to come from zero or near-zero fuels and technologies by 2030, with a stretch goal of 10%.4Ocean Engineering. Evaluation of techniques to reduce carbon emissions from ships within the scope of revised greenhouse gas emission targets for 2030, 2040, and 2050
On a regional level, the European Union began including shipping in its Emissions Trading System (EU ETS) starting in 2024. Studies modeling an EU maritime carbon price estimated that even at a relatively low price of ten to fifty dollars per tonne of CO2, the shipping industry could contribute up to 0.3–1.4 billion dollars per year in carbon allowance costs, while creating financial incentives for specific abatement technologies such as wind-assistance systems.5Transportation Research Part D: Transport and Environment. The impact of an EU maritime emissions trading system on oil trades That money changes the math for shipowners deciding whether an expensive retrofit is worth it.
Operational Measures You Can Deploy Now
Not every solution requires a new fuel or a new engine. Some of the quickest wins come from running existing ships more intelligently. Weather-informed route optimization, where algorithms adjust a vessel’s path based on sea state, currents, and wind forecasts, can achieve reductions of more than 7% on average in fuel consumption, travel distance, and journey time.6Ocean Engineering. Integrating weather-informed routing and energy optimization for sustainable maritime transportation That sounds modest, but applied across tens of thousands of voyages per year, the cumulative fuel savings are enormous.
Reducing friction between the hull and the water is another lever. Air lubrication systems pump a thin layer of air bubbles, a continuous air film, or a trapped air cavity beneath the hull to lower drag. A global fleet analysis found net power savings of 2–5% from air bubble systems, 8–14% from air layer systems, and 16–22% from air cavity technology under calm-water conditions.7International Journal of Naval Architecture and Ocean Engineering. Potential energy savings of air lubrication technology on merchant ships The technology is still maturing and performance drops in rough seas, but several large shipping companies are already installing air lubrication on new-build vessels.
Wind-assisted propulsion is also making a quiet comeback. Modern implementations include rigid wing sails, Flettner rotors (tall spinning cylinders that exploit a physics effect to generate thrust), and towing kites. Each has different strengths depending on the ship type and route, and real-world fuel savings vary widely with wind conditions. The technology works best as a supplement rather than a replacement for engine power, shaving fuel burn on favorable routes while adding relatively little cost or complexity to the ship.
The Alternative Fuel Landscape
The biggest long-term question for shipping emissions is what replaces the heavy fuel oil and marine diesel that currently power nearly the entire fleet. Several candidates are in various stages of readiness, and none is a clear winner.
Liquefied Natural Gas
LNG is the most commercially mature alternative. Switching from marine gasoil to LNG on a cruise ship with newer dual-fuel engines showed roughly a 21% drop in CO2-equivalent emissions on a well-to-wake basis.8Journal of Marine Science and Engineering. Methane Slip, Black Carbon and Greenhouse Gas Emissions from an LNG-Fuelled Cruise Ship However, LNG’s climate benefit is undermined by methane slip: unburned methane escaping from the engine. Methane is a far more potent greenhouse gas than CO2 over shorter time horizons. While newer engine generations have reduced methane slip at higher loads, it spikes at lower loads. An engine running at about a quarter of its capacity showed methane slip roughly triple what it produced at cruise loads.8Journal of Marine Science and Engineering. Methane Slip, Black Carbon and Greenhouse Gas Emissions from an LNG-Fuelled Cruise Ship LNG is best understood as a transitional fuel rather than a destination: better than heavy fuel oil, but nowhere near zero-emission.
Green Methanol
Methanol produced from renewable electricity, captured CO2, and water (“green methanol”) is attracting serious investment. Major container lines have already ordered methanol-capable ships. A modeling study found that offshore wind-powered green methanol could become cost-competitive with conventional marine fuels after 2030 and is expected to be uniformly cheaper by 2035, while fully meeting EU regulations for renewable fuel content.9Nature Communications. Cost-competitive offshore wind-powered green methanol production for maritime transport decarbonization Methanol is easier to handle than ammonia or hydrogen, since it is a liquid at room temperature and already has established bunkering infrastructure in some ports. Its energy density is lower than conventional fuels, meaning ships need larger fuel tanks, but the trade-off in cargo capacity is manageable for most vessel types.
Ammonia
Ammonia burns without direct CO2 emissions and can be produced from green hydrogen and nitrogen. On paper, it is an appealing zero-carbon fuel. In practice, safety is the dominant concern. Ammonia is classified as a toxic gas, and even moderate concentrations can be fatal with prolonged exposure. Risk assessments show that individual crew risk inside enclosed fuel preparation rooms on ammonia-powered ships is one to one-and-a-half orders of magnitude higher than in equivalent LNG systems.10Journal of Loss Prevention in the Process Industries. Safe horizons: A critical review of the human, technical, and regulatory challenges/opportunities for ammonia as a marine fuel Bunkering requires significantly larger safety exclusion zones because toxic ammonia plumes remain hazardous at distances far beyond where LNG disperses safely.10Journal of Loss Prevention in the Process Industries. Safe horizons: A critical review of the human, technical, and regulatory challenges/opportunities for ammonia as a marine fuel These are not unsolvable problems, but they require fundamentally new safety protocols, crew training, and port infrastructure before ammonia-fueled ships can operate at scale.
Hydrogen
Hydrogen offers zero carbon at the point of use, whether burned in an engine or fed through a fuel cell. The challenge is volume. Hydrogen has excellent energy per kilogram but terrible energy per cubic meter, even as a cryogenic liquid. Modeling of a deep-sea vessel showed that a minimum viable hydrogen system using liquid storage and fuel cells would require roughly 8,900 cubic meters, comparable in total system volume to an ammonia-based setup and significantly larger than a methanol system at about 6,000 cubic meters.11International Journal of Hydrogen Energy. Hydrogen as a deep sea shipping fuel: Modelling the volume requirements Hydrogen is a realistic option for shorter routes and smaller vessels, but for the largest ocean-going ships, the sheer tank volume eats into cargo space in ways that make it hard to compete commercially.
Capturing Carbon Onboard
Instead of switching fuels, some researchers are exploring fitting ships with onboard carbon capture equipment that strips CO2 from the exhaust and stores it as a liquid for offloading at port. The idea is appealing for the existing fleet since it lets conventional engines keep running while dramatically cutting stack emissions. But the engineering hurdles are serious.
A solvent-based capture unit operating above a 50% capture rate on a ship burning LNG needs more thermal energy than the engine exhaust alone can provide. An afterburner has to make up the shortfall, increasing total fuel consumption by an estimated 6–9% for LNG and 8–12% for diesel.12International Journal of Greenhouse Gas Control. Energy assessments of onboard CO2 capture from ship engines by MEA-based post combustion capture system with flue gas heat integration Then there is the weight and space problem. A study of retrofitted container ships estimated that the capture unit and stored CO2 would consume up to 8% of a methanol ship’s deadweight capacity and roughly 10% of its container space, while achieving a net carbon capture efficiency of about 91%.13ACS Sustainable Chemistry & Engineering. Onboard Carbon Capture for Circular Marine Fuels The cost of capturing that CO2 ranged from roughly 140 to 350 dollars per tonne, and total capital expenditure reached tens of millions of dollars per vessel.13ACS Sustainable Chemistry & Engineering. Onboard Carbon Capture for Circular Marine Fuels
On top of all that, liquefying the captured CO2 for storage is itself an engineering challenge. Some system designs require temperatures and energy inputs that are impractical on a ship, leaving only a few feasible configurations for real-world deployment.14Journal of Cleaner Production. Onboard carbon capture and storage (OCCS) for fossil fuel-based shipping: A sustainability assessment Onboard carbon capture is technically possible, but at current costs and space penalties, it is a niche solution rather than a fleet-wide strategy.
Shore Power and Port-Side Emissions
Ships do not just pollute while sailing. At berth, vessels run auxiliary engines to power onboard systems like lighting, refrigeration, and cargo handling. Cold ironing, the practice of plugging into shore-based electrical power while docked, can eliminate those auxiliary engine emissions entirely.15Transportation Research Part A: Policy and Practice. Prospects of cold ironing as an emissions reduction option The climate benefit depends on how clean the local electrical grid is: a ship plugging into a coal-heavy grid might simply move its emissions from the port to the power plant. But in ports served by renewable or low-carbon grids, shore power eliminates port-side CO2, nitrogen oxides, sulfur oxides, and particulate matter in one stroke. Research on the Spanish port system quantified the external costs from CO2, NOx, SOx, and particulate emissions from berthed ships, underscoring that the largest ports have the most to gain from cold ironing investment.16PubMed. Potential of cold-ironing for the reduction of externalities from in-port shipping emissions: The state-owned Spanish port system case
The main barriers are infrastructure cost and standardization. Shore power installations require heavy electrical connections, transformer capacity, and compatible plug systems on the ship side. Many older vessels are not equipped. Regulatory pressure is helping: California already mandates shore power at major berths, and the EU is phasing in requirements for its largest ports.
The 25-Year Fleet Problem
Even if the perfect zero-emission ship existed today, the global merchant fleet could not turn over quickly enough to meet 2050 targets. Commercial vessels are built to sail for 25–30 years, and construction takes two to three years per ship. The fleet’s massive inertia means that vessels ordered today will still be operating in the 2050s.17Transportation Research Interdisciplinary Perspectives. Modelling the shipping transition: Forecasting merchant fleet emissions to 2050 This reality forces the industry to pursue two tracks simultaneously: design new ships for alternative fuels, and retrofit existing ones with whatever incremental efficiency improvements are available. A vessel launched in 2025 burning conventional fuel might receive a wind rotor, an air lubrication system, and route-optimization software over the next decade, each chipping away a few percentage points of fuel use.
Fleet modeling makes clear that waiting for a single breakthrough technology is not a viable strategy. The math only works if the industry stacks multiple partial solutions on top of each other across both new and existing vessels.
Scrubbers and the Sulfur Question
An adjacent debate involves sulfur emissions. The IMO’s 2020 sulfur cap required ships to either use low-sulfur fuels or install exhaust gas cleaning systems known as scrubbers, which strip sulfur from the exhaust. A comprehensive life-cycle assessment comparing a bulk carrier running heavy fuel oil with a scrubber against the same ship burning low-sulfur alternatives found that, when particulate matter abatement is in place, heavy fuel oil with a scrubber performs about the same as marine gasoil and actually outperforms very-low-sulfur fuel oil in several environmental impact categories.18PubMed Central. Marine Scrubbers vs Low-Sulfur Fuels: A Comprehensive Well-To-Wake Life Cycle Assessment Supported by Measurements Aboard an Ocean-Going Vessel This matters because scrubbers have been criticized for dumping acidic washwater into the ocean. The environmental picture turns out to be more complicated than the simple narrative of scrubbers being a dirty cheat: their full life-cycle performance depends on how you weight air quality gains against water quality costs.
Black Carbon and Warming Beyond CO2
CO2 is the headline number, but ships also emit black carbon, essentially soot, which has an outsized warming effect in sensitive regions. The Arctic is warming nearly four times faster than the global average, and black carbon from maritime transport accelerates ice melt and amplifies both regional and global climate impacts.19Climate Law. Mitigating Arctic Maritime Black Carbon Emissions Through Fuel-Based Regulation: A Critical Appraisal When soot lands on ice or snow, it darkens the surface and causes it to absorb more sunlight. Arctic shipping routes are opening up as sea ice retreats, which paradoxically increases maritime traffic and black carbon deposition in the region most vulnerable to it. Fuel-based regulations targeting black carbon are still in early stages, and the IMO has so far relied on voluntary measures for Arctic shipping. Some researchers argue that fuel switching to distillate fuels or LNG in polar waters would significantly cut black carbon, but enforcement in international waters remains difficult.
The Nuclear Question
Nuclear propulsion has powered military submarines and aircraft carriers for decades, and a handful of experimental merchant vessels were built in the mid-twentieth century. Analysis of those historical experiments found that nuclear propulsion is technically feasible for commercial shipping and that nuclear fuel is cheap enough to make high-speed operations economically viable in a way conventional fuel cannot match.20Journal of Cleaner Production. Nuclear propulsion in ocean merchant shipping: The role of historical experiments to gain insight into possible future applications But the barriers are enormous. Port and canal access for nuclear-powered merchant ships was restricted even during the experimental era, and upfront construction costs, refueling, and end-of-life decommissioning expenses are vast and unpredictable compared to conventional vessels.20Journal of Cleaner Production. Nuclear propulsion in ocean merchant shipping: The role of historical experiments to gain insight into possible future applications Public acceptance, insurance liability, and the geopolitical implications of nuclear reactors moving through international waters add further layers of difficulty. Small modular reactor designs have revived interest in the concept, but commercial nuclear shipping remains speculative rather than imminent.