A single large cargo ship burns through tens of thousands of tons of fuel per year and can emit roughly 50,000 to over 100,000 metric tons of CO2 annually, depending on its size, speed, and route. Globally, maritime shipping produced 973 million metric tons of CO2 in 2024, up 9.4% from 2019. That makes the shipping industry responsible for roughly 2.5% to 3% of all human-caused carbon emissions.
Those numbers sound enormous, but they come with context: ocean shipping moves over 100 trillion tonne-kilometers of freight each year, making it by far the most carbon-efficient way to transport goods. The real picture depends on the type of ship, how fast it’s moving, what fuel it burns, and how much cargo it’s actually carrying.
Emissions Per Container, Per Mile
The most useful way to measure a cargo ship’s emissions isn’t total output but carbon intensity: how much CO2 it produces for each unit of cargo moved a given distance. For container ships, the average is about 150 grams of CO2 per twenty-foot container (TEU) per nautical mile. That means shipping a single standard container from Shanghai to Los Angeles, roughly 5,600 nautical miles, produces about 840 kilograms of CO2.
But that average hides a wide gap. The cleanest ships emit under 100 grams per TEU per nautical mile, while the dirtiest exceed 200 grams. Across all major ship types, the International Council on Clean Transportation has found a 2.5 to 5 times difference in carbon intensity between the best and worst performers carrying the same cargo the same distance. Ship age, engine efficiency, hull condition, and how full the ship is all drive that gap.
How Ships Compare to Trucks and Planes
Despite their massive total emissions, cargo ships are the most efficient freight movers on the planet. The numbers from MIT’s Climate Portal illustrate this clearly: sea and inland waterway shipping moved over 101 trillion tonne-kilometers of freight in a recent year while producing 657 million tonnes of CO2. Road freight moved about a quarter of that cargo but emitted 2,230 million tonnes, more than three times as much. Air cargo is in another league entirely, moving just 303 billion tonne-kilometers while producing 155 million tonnes of CO2.
Put differently, trucking can emit more than 100 times as much CO2 as a ship to carry the same amount of freight the same distance. Air freight is even worse by comparison. So while a single cargo ship’s annual emissions rival those of tens of thousands of cars, it’s also replacing what would otherwise be far more polluting transport by road or air.
What Fuel a Ship Burns Matters
Most large cargo ships still run on residual fuel oil, the thick, heavy byproduct left over after refining crude oil. It’s cheap but dirty, producing about 11.27 kilograms of CO2 per gallon burned. Marine diesel (similar to standard diesel fuel) is somewhat cleaner at 10.21 kg of CO2 per gallon. Liquefied natural gas (LNG), which a growing number of newer vessels use, drops to 4.46 kg of CO2 per gallon, less than half the carbon output of heavy fuel oil per unit of fuel.
That fuel switch doesn’t translate to a full 60% emissions cut in practice, because LNG has a different energy density and engines using it have different efficiency profiles. There are also concerns about methane leakage during LNG handling, which partially offsets the CO2 advantage. Still, the fuel type is one of the biggest single variables in a ship’s carbon footprint.
Speed Has a Dramatic Effect
One of the simplest ways to cut a cargo ship’s emissions is to slow it down. The relationship between speed and fuel consumption isn’t linear; it’s closer to a cube function, meaning small speed reductions yield outsized fuel savings.
Reducing a container ship’s speed by just 10% cuts CO2 emissions by about 19 to 21%. A 20% speed reduction drops emissions by roughly 34%. And slashing speed from 23 knots down to 14 knots, a reduction of about 39%, cuts fuel consumption per nautical mile by over 61%. At extreme slow steaming, where a ship designed for 23 knots crawls at 12 knots, fuel consumption drops by 72% to 76%, producing roughly one quarter of the CO2 it would at full speed.
The trade-off is time. That 12-knot crawl nearly doubles the voyage duration compared to sailing at 23 knots. For perishable goods or time-sensitive supply chains, that’s a real cost. But for bulk commodities like iron ore, coal, or grain, slow steaming is widely practiced and has become standard operating procedure since fuel prices spiked in the late 2000s.
How Ships Are Rated for Carbon Intensity
Since 2023, the International Maritime Organization (IMO) has required commercial vessels to carry a Carbon Intensity Indicator (CII) rating. Ships are graded A through E based on how much CO2 they emit per unit of cargo carried per nautical mile. An A rating means the ship significantly outperforms its class average; an E means it’s among the worst.
This isn’t just a label. A ship rated D for three consecutive years, or E in any single year, must submit a corrective action plan showing how it will improve to at least a C. That can mean engine upgrades, operational changes, or in some cases, early retirement of the vessel. The system creates a financial incentive too: charterers increasingly prefer higher-rated ships, and a poor rating can reduce a vessel’s market value.
Technology Gains Are Modest So Far
Shipbuilders and operators are adding energy-saving devices like propeller ducts, air lubrication systems, and wind-assisted propulsion (rigid sails or rotor sails mounted on deck). The efficiency gains from these technologies range from 2% to 15% depending on the device and vessel type. Ships fitted with two energy-saving devices outperformed their class average by about 3%, and those with three devices by about 6%.
These are real but incremental improvements. A propeller duct, which reshapes water flow into the propeller to generate more thrust per unit of fuel, costs roughly $500,000 on a large bulk carrier and delivers 2% to 9% lifetime efficiency gains. Stacking multiple technologies helps, but no single retrofit comes close to the 20% to 40% reductions achievable through slow steaming alone.
Where the Industry Is Headed
The IMO’s 2023 greenhouse gas strategy sets two major targets: reduce total annual shipping emissions by at least 20% (striving for 30%) by 2030 compared to 2008 levels, and reach net-zero emissions by or around 2050. Given that emissions have risen nearly 10% in just five years, hitting even the 2030 target will require aggressive action across the fleet.
Meeting these goals will likely require a combination of everything discussed above: slower speeds, cleaner fuels, better hull and propeller technology, larger and more efficient ships, and eventually zero-carbon fuels like green ammonia or green methanol. For now, the global cargo fleet remains overwhelmingly powered by fossil fuels, and its total emissions are still climbing.