As we all know from rhymes at school, “in fourteen hundred and ninety-two, Columbus sailed the ocean blue”, and since then a very large number of ships have crossed the Atlantic and the other seven seas. Unlike the Santa Maria, in the 20th century the vast majority of them have been powered by a fossil fuel of some kind. On Wednesday I read an article in the Guardian about an idea that could change that, at least to some extent. Putting sails back on ships could be a way to make them greener, and crucially for the shipping companies, consume less fuel (which like all fossil-based fuel in the world is becoming increasingly expensive).

Companies such as Skysails are promoting this technology as a cost-saving measure for freight shipping companies, with the slogan “Turn wind into profit” (which reminds me of an amusing advert). Because of the significant contribution to climate change caused by shipping (due to the large number of goods we trade around the world), making freighting by greener must be a good thing (although localising economies so less stuff is shipped is a key part of the solution too).

But how green is cargo ship travel for passengers? Various people have taken this option as an alternative to flying (including Kate, the founder of Loco2; you’ll be able to read blogs about the experience soon) but as yet there is little information available about the CO2 emissions per person because of the fact that the ships are (of course) primarily carrying cargo, not people. This makes it difficult to ascertain the contribution of individual passengers to the overall carbon emissions.

One argument (discussed here) says that because the ship is travelling anyway to get the cargo to its destination, the passengers’ travel is irrelevant, and therefore the journey can be seen as carbon neutral. That’s all well and good whilst we’re talking about a small number of pioneers taking the opportunity to have a trans-Atlantic adventure, but it’s not going to work for large number of passengers (the main risk involved with scaling up passenger numbers is that we’ll go down the polluting cruise ship route).

A way to look at cargo ship travel that takes into account the passenger contribution is to work it out in terms of weight. This is the method that the mighty Barbara Hadrill used to calculate the emissions on her massive adventure to Australia. Barbara worked out that the freight ship leg of her adventure (Singapore – Darwin) produced 285,760 grams CO2 for 2350 miles (see her August 2006 blog).

Calculating in terms of weight is far from an established method, but it is logical, and should help us to think about the comparative advantages of each mode of transport, both in terms of CO2 and fuel efficiency (which is an increasing concern for all of us given massively rising energy costs).

Right off the top of my head, the method goes something like this (I may come back and improve on this as I talk to people about it):

• Think about a humble passenger cabin on a cargo ship where all other space is used as efficiently as possible (because the more cargo the ship holds, the more cost-effective the freighting). This is going to be far better in CO2 terms than a decadent cruise ship where not only the cabin, but the whole ship is engineered around comfort and entertainment, rather than simply getting as much cargo (human or otherwise) from A to B on as little fuel as possible. For a cruise ship calculating the CO2 emissions per person is simply a case of dividing the total emissions by the number of passengers (as that’s the only purpose of the voyage), but for a cargo ship we need to take into account the fact that it’s carrying cargo as well.

• Let’s assume a cargo ship can carry 3000 tonnes of cargo (that’s 3 million kg) and that a person weighs 65kg (about 10 stone). In this situation (if my calculations are correct) the passenger would be responsible for 0.002% of the overall CO2 emissions of the journey. I couldn’t find a fuel consumption figure for a typical trans-Atlantic journey (the nearest I got was here) but we can safely bet that 0.002% of it isn’t very much. Now obviously this assumes that to carry everyone that efficiently we’d need to cram all the passengers into containers like they were cargo, and that would be mental, but it does give us a useful indication of the terms we can view the issues in.

The basic conclusion (and we probably didn’t need to do such ludicrous calculations to work it out) is that using space more efficiently when travelling is a good thing. What it doesn’t tell us unfortunately, is a comparable CO2 per km figure for passenger cargo-ship travel versus air travel (or other means). There’s still work to be done on this, and I’ll be continuing to dig around places like the New Scientist and sustainable shipping sites to try and find more answers.

In the meantime, I’ll leave you with two thoughts:

• There needs to be a clear distinction between greenhouse gases that cause climate change (a very pressing global concern) and other pollutants such as Sulphur Dioxide that cause local environmental damage (an immediate concern for some, but not something that will affect all of us).
• We need to think about the feasible efficiency advances available in shipping/ferries (such as solar power) versus the feasible efficiency advances available in aviation (such as airships) and then make bold decisions about mass transportation on that basis.

Coming soon: smaller boats powered by the wind and the sun…

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