Sutton Coldfield, West Midlands, UK
If the world’s ocean-going commercial fleet were removed from the sea, global mean sea level would quickly fall, but by only a matter of micrometres.
By Archimedes’ principle, a floating vessel displaces enough water for the mass of that water to equal the combined mass of the vessel and everything aboard it. This means that a ship with a total mass of 100,000 tonnes displaces 100,000 tonnes of seawater, equivalent to about 97,560 cubic metres, given a seawater density of approximately 1.025 tonnes per cubic metre.
The ship itself is represented by its lightship mass: the mass of its hull, machinery and permanent equipment. Deadweight tonnage (DWT) is the maximum additional mass that a vessel is permitted to carry, including cargo, fuel, fresh water, ballast, stores and people. Ballast is used to provide stability, control draft and trim, and keep the propeller and rudder adequately immersed.
Removing the ships would buy us less than a day’s reprieve from the current sea-level rise of 4.5 millimetres a year
According to the UN Conference on Trade and Development’s 2025 Review of Maritime Transport, there are about 112,500 propelled seagoing merchant vessels of at least 100 gross tonnage (GT), with a combined carrying capacity of 2.44 billion DWT. GT is a dimensionless measure based on a ship’s size (specifically its enclosed volume), not its mass. Maximum displacement is greater than this because it also includes the vessels’ lightship mass. Using typical ratios of deadweight to displacement for different classes of merchant ship suggests that total maximum displacement is about 3 billion tonnes, equivalent to about 2.9 cubic kilometres of seawater.
Given that Earth’s radius is about 6400 kilometres and the ocean covers about 70 per cent of its surface, the ocean has a surface area of approximately 360 million square kilometres. Dividing the displaced volume by this area gives a maximum global mean sea-level rise of about 8 micrometres.
The actual figure would be somewhat lower than this because the entire fleet is never simultaneously carrying its maximum deadweight.
Here’s a back-of-the-envelope calculation. A quick internet search tells me that the carrying capacity of the global commercial shipping fleet is about 2.5 billion deadweight tonnes, or about 2.25 billion metric tonnes. Let’s assume a total of 3 billion tonnes for all ships and boats.
That mass equates to 3 cubic kilometres of water. On the other hand, the total ocean volume is 1.3 billion cubic kilometres.
Hence, removing the ships is going to remove less than half of one part per billion of the total ocean volume. The average depth of the ocean is roughly 3700 metres. Removing half a billionth of that drops the surface level by roughly 18 micrometres. Human hairs typically have a thickness of between 30 and 60 micrometres.
I don’t think anyone will notice a sea-level drop of half the thickness of a hair. It certainly won’t help to counter the effects 0f global warming.
The total global tonnage of ships is around 2.5 billion deadweight tonnes (fully loaded). Some will be in dry dock and many won’t be fully loaded, and a deadweight tonne doesn’t quite equal a tonne, or 1 cubic metre of water. There is also a long tail of countless tiny fishing boats on the sea. So, approximately 2.5 billion cubic metres of water are displaced by these ships. However, the world has 361 million square kilometres of ocean, so removing all these ships would result in a sea-level drop of just 7 micrometres, which would be almost instantaneous as the world’s ships are fairly evenly spread across the seas, outside Arctic circle latitudes.
That would buy us less than a day’s reprieve from the current sea-level rise of 4.5 millimetres a year. We might gain more by removing the 2.5 per cent of global carbon dioxide emissions accounted for by shipping, but minus the extra CO2 from transport that shifts to road or rail, for instance in cargoes from East Asia to Europe. Also, a lot of oil is shipped by sea, so CO2 might fall even more as petrol and diesel gets very scarce and expensive in Europe and other places.
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