When I travel, I try to limit my environmental impact. I don't want to refrain from travelling completely, but when I do travel, I tend to favour ground-level transportation over air-transportation, if distances are not too large (e.g., if traveling over-land would take more than three days, I might fly anyway.
A well-filled train usually has a much lower ecological footprint per traveller-kilometer than an æroplane, particularly if the train is hydro-electrical, such as in Sweden. High speed trains and diesel trains are already worse, but still considerably better than flying. But what about ferries? Fuel used by (fast) ferries can be quite dirty. On the other hand, ferries may carry over a thousand travellers, sometimes several thousands.
How does the ecological footprint per traveller of a typical, well-filled ferry compare to the ecological footprint per traveller of an æroplane? For the sake of this question, I'm interested in ferries going out on the open ocean on routes that potentially compete with airplanes, not in local ferries crossing a river, a lake, a fjord, or a small sound (there's probably no mode of transportation spanning as many orders of magnitude in size as a boat).
For example, travelling from Stockholm to Warsaw, one might identify four alternatives with a somewhat similar travel class, where all day trains are 2nd class and all overnight accommodation (train or ferry) is with a bed/berth in a shared cabin.
- flying in an ordinary economy seat
- by train+ferry via Nynäshamn–Gdańsk
- by train (almost) all the way via Copenhagen, Hamburg, Berlin (a considerable detour)
- (only part of the year) by train-on-ferry via Malmö and Berlin (Berlin Night Express)
Which one is the most ecological?