I've been reading about the Potosi, a five-masted square-rigged ship built in 1895 with a steel hull. The best Internet link I can find is here. Yes, the weather is still dreadful here so I'm not going out.
This ship weighed 4000 tons. Proper British tons, not those French tonnes we have to use now. It could carry 6000 tons of cargo so when at sea it was ten thousand tons of stuff and all the sail meant it could do 14 knots and surprise a steamer or two that tried to cut across its bows.
It sailed from Germany to America, round the bottom of Africa and on to India and back to Europe twice a year.
Guess how much fuel it used?
None at all. It had no engine. It was entirely powered by sail.
Now, I'm not going to say that all those container ships and oil ships must do the same even though six of them produce more pollution than all the cars in the world, but consider...
The container and oil ships are huge. Far longer and wider than Potosi. They could have a few masts and sails and just think - when the wind's in the right direction they'd need no fuel. They could travel for free. When the wind is going the wrong way they could use their engines but imagine - free transport for a lot of the trip.
Go on, ship owners. You know you want to think about it at least.
Stuff the carbon trading. There is a way to transport your goods for free and in the current world climate you'll get massive kudos for doing it.
It'll make the ships more interesting too.