Shipping: Why Distance Stopped Mattering
Almost everything in a European home arrived at some point by sea. Shipping is invisible in daily life and is roughly three percent of global energy use and a similar share of emissions - a small number that moves about eighty percent of world trade by volume.
Why Ships Are So Efficient
A truck's engine does two jobs: it supports nothing, since the road does that, but it must overcome rolling resistance and accelerate a heavy vehicle repeatedly. A ship's hull is supported entirely by buoyancy, so the engine's only task is to overcome the resistance of water and air at a steady speed.
Scale then compounds the advantage. Drag depends roughly on surface area while cargo capacity depends on volume, and volume grows faster than area as a vessel gets larger. A ship twice as long in every dimension carries eight times the cargo with about four times the wetted surface, which is why container ships kept growing and why the largest ones are the most efficient per tonne.
The result in numbers: a large container ship moves a tonne of cargo a kilometre on a few grams of fuel. Rail is several times worse, road freight roughly fifty times worse, and air freight around a hundred times worse than rail - which is why air freight is reserved for goods where time dominates value.
The economic consequence is the one that shaped the world. When ocean transport costs almost nothing per unit of goods, the cost of distance nearly vanishes from a manufacturing decision, and production locates wherever labour, capital, regulation and energy prices are most favourable. Containerisation, which cut handling costs as dramatically as ships cut transport costs, completed the effect.
Speed, and the Cube Law
Resistance through water rises steeply with speed - roughly with the square for the drag force, and therefore roughly with the cube for the power required. This makes speed the single most powerful lever a shipping operator has.
The 2008 financial crisis demonstrated it at scale. With freight rates collapsed and fuel expensive, operators adopted slow steaming, reducing service speed substantially. Fuel consumption per voyage fell far more than proportionally, and the practice persisted long after the crisis because the economics stayed favourable.
The trade-off is capital and inventory. A slower ship makes fewer voyages a year, so the same trade needs more vessels, and goods spend longer in transit as working capital. Slow steaming is therefore cheapest when fuel is expensive and ships are idle, which is exactly when it was adopted.
It remains among the largest available emissions reductions in the sector, achievable with the existing fleet and no new technology. That it is not applied more widely is a function of freight rates and delivery expectations rather than of engineering.
The Fuel, and What Happened in 2020
Ships have historically burned heavy fuel oil, the residue left after lighter products are distilled out of crude. It is cheap, viscous enough to need heating before use, and high in sulphur.
In 2020 the International Maritime Organization cut the permitted sulphur content of marine fuel from 3.5 percent to 0.5 outside designated control areas. Operators responded by switching to low-sulphur fuel, fitting exhaust scrubbers, or moving to distillate. The regulation substantially reduced sulphur emissions and the associated air pollution, which was its purpose and a clear public health gain.
It also produced an effect nobody had highlighted in advance. Sulphate particles from ship exhaust had been seeding brighter, longer-lived marine clouds that reflected sunlight - an unintentional cooling that partly masked warming over shipping lanes. Removing the sulphur removed the masking, and researchers have since attributed a measurable warming contribution to the change.
The episode is worth stating carefully because it is easily misused. It is not an argument against cleaning up shipping fuel: the sulphur was causing tens of thousands of premature deaths a year, and the cooling was an accidental side effect of that harm. It is a demonstration that the climate and air quality effects of a fuel are distinct, can run in opposite directions, and need to be counted separately - which is exactly what a proper life cycle assessment does.
What Ships Will Burn Next
Batteries work for ferries and short coastal routes and do not scale to ocean crossings, for the same energy density reason that constrains aviation - though less severely, since a ship can carry more mass than an aircraft.
Ammonia is the most discussed candidate. It contains no carbon, is liquid at modest pressure or moderate cooling, and can be made from hydrogen and nitrogen using existing industrial processes. Against it: ammonia is toxic, requires careful handling in ports and at sea, and burns poorly enough to need a pilot fuel. Nitrogen oxide emissions and nitrous oxide slip are unresolved concerns.
Methanol is the pragmatic alternative. It is liquid at ambient conditions, handled routinely in existing infrastructure, and burns cleanly in modified engines. It contains carbon, so it only helps if made from biomass or captured carbon dioxide rather than from natural gas. Several large operators have ordered methanol-capable vessels, which is the strongest signal available about where the industry expects to land.
Wind assistance has returned as a serious proposition rather than a curiosity. Rotor sails, rigid wings and kites can cut fuel use by a meaningful percentage on favourable routes, they work with any fuel, and they retrofit to existing ships. In a sector where the fleet turns over across decades, anything that improves vessels already built is worth more than its percentage suggests.
Frequently asked questions
Why is shipping so energy-efficient?
Because buoyancy supports the cargo, so the engine only overcomes water and air resistance at steady speed rather than supporting and repeatedly accelerating a heavy vehicle. Scale compounds this: cargo capacity grows with volume while drag grows with surface area, so larger vessels are more efficient per tonne.
How does shipping compare with road and air freight?
A large container ship moves a tonne of cargo a kilometre on a few grams of fuel. Rail is several times worse, road freight roughly fifty times worse, and air freight around a hundred times worse than rail, which is why air freight is reserved for goods where time dominates value.
What is slow steaming and why does it save so much?
Reducing service speed. Power required rises roughly with the cube of speed, so a modest speed reduction cuts fuel consumption far more than proportionally. It was adopted widely after 2008 and remains among the largest available emissions reductions, achievable with the existing fleet and no new technology.
What happened when ship fuel sulphur was cut in 2020?
Sulphur emissions and the associated air pollution fell substantially, which was the purpose and a clear health gain. It also removed sulphate particles that had been seeding brighter marine clouds, an accidental cooling that had partly masked warming over shipping lanes, and researchers have attributed a measurable warming contribution to the change.
What fuel will replace heavy fuel oil?
Ammonia and methanol lead. Ammonia contains no carbon and is liquid at modest conditions but is toxic, burns poorly and has unresolved nitrogen emissions. Methanol is liquid at ambient conditions and handled in existing infrastructure but contains carbon, so it only helps if made from biomass or captured carbon dioxide.
