Goodness knows, Keith. LNG is considered a clean alternative; four of my ships were LNG tankers that used the cargo boil-off to fire the steam boilers, and very good it was too. Trouble is, it's still a fossil fuel. As for wind power, I can't see these huge modern ships ever carrying enough sail to move them along.
There doesn't seem to be an easy answer for aircraft at the moment, does there. We'll just have to go back to trans-Atlantic liners.
Maybe a shift away from large ships to trans / inter-continental railways is a possibility. Theoretically, you can already travel by land from the Bering Sea coast of Russia and China to the Middle East, down through Africa to Cape Town, and to almost anywhere in Europe. Connect Alaska to Russia, and then you can go all the way down to Cape Horn. Link up Indonesia through to New Guinea and then Australia by tunnel. Do all that, and the gaps can be filled in by much smaller vessels than we have now, acting like 'feeder' ships to and from the main rail terminals. We'd still need fossil fuel-powered aircraft for remote islands, but the global rail system would be powered by electricity from renewable sources.
There you go, all sorted - shouldn't cost too much.
