europe - australia by ship

This site gives some insight into emissions produced by cargo ships.

#12: i did not mis-place a decimal point.
according to www.atmosfair.de, a german website, it is about 12.000 (twelve thousand) kg!

and another thing you have to consider: emissions caused by airplanes are much more harmful than emissions from ships, cars etc., cause they are let off (sorry, i dont know how to correctly in english) at much higher altitudes and thus directly cause harm in the atmosphere.

What about the fact that the ships fuel is less refined
Our resident petrochemical engineer would have a better idea but from what I understand diesel is part of the same fractional distillation process that produces aviation kerosene and so has much the same energy input. If you didn't produce kerosene then you could lower your distillation temperatures to only produce diesel but that isn't what happens.

The ship also has a dual purpose in carrying cargo AND passengers
The ships to OZ don't really have a dual purpose, the passengers they carry are more like your mailbag example, an add on. I don't deny that, as your last link mentioned, those passengers are freeloading on CO2 that would have been generated anyway but the more people that chose this method the more demand for passenger space on cargo ships there will be until you loose the benefit of freeloading on a cargo ship.

The more you can stack things the more efficient your chosen mode of transport is. Slow cargo ships are great for heavy cargo that doesn't need to get anywhere fast. The cargo can be stacked and forgotten about. Planes are great for people because of the short journey time you can pack them relatively tightly and gain efficiencies, this is the premise of the A380. If you put the same amount of people on a ship you need to give them more room and so loose efficiency because you now have a lot more things you need to move around for them. You end up with a cruise ship which as already mentioned are worse for CO2 than long haul flights.

Good points about cargo stacking. Planes generally have a pre-defined route, and therefore travel that route wether full, half full or empty. the emissions people are talking about are of the presumption the plane is full, which quite often isnt the case. So your ratio for fuel burnt per customer obviously goes up when less passengers are flying.
Ships, especially Tramps and Container vessels, will go from port to port in order to ensure a full load. If we stick to the cargo ship carrying passengers debate, the life of a ship and plane are about the same, give or take 5 years. A ship goes into dry dock maybe once each 4 years, for around 4 to 6 weeks. A plane goes in for a D check maybe once every 2 or 3 years, depending on hours flown. A host of variables now present themselves, such as operating costs beyond just fuel, maintenance costs, infrastructure required for efficient operations etc. I have a headache just thinking of which could be deemed the less environmentally damaging mode of transport.

Plus the plane will take off anyway more than likely with someone in your place.

Going by ship from Europe would be an almighty task, but travelling on the Trans-Siberian railway into SE Asia and getting a ride on a cargo ship is entirely possible. It'll take at least a month, but it's something I plan to do next year because it's such an awesome journey.

This site will get you started: Seat 61

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<hr>So your ratio for fuel burnt per customer obviously goes up when less passengers are flying. <hr></blockquote>

So there is an argument for flying, in a convoluted way.

A convoluted argument I mean, not for flying convolutedly.

Plane emmisions are worse than many emmisions, due to the fact that they hang out up in the higher part of the atmospehre ( I did read that, it must be true).

But a plane will fly with or without you, ditto the ship. Of course if no one travelled, this would not be true- but I am trying to keep the argument realistic.

I think if you want to be environmentally correct and stil travel either a) take a sailing ( wind ) ship or b ) buy those carbon offset thingies.

I've found the answer.
We can travel by kite and save the universe.

And before the pedants jump on me.--- Yes, I know it only saves 10 -35% fuel but hey! it looks pretty cool.

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<hr>walk, sail or cycle.<hr></blockquote>
Does one need to factor time taken into the equation? Let us compare a short trip - Melbourne to Sydney (why anyone would choose to go in that direction is another issue altogether). By air it takes about an hour of flying time plus about 15-20 minutes taxiing - walking would take about 3 weeks. Do we need to factor in the resources required to support a person for 3 weeks, and the CO2 expired in that time when comparing the 2 modes of transport?

And don't mention carbon credits or carbon trading - just a scheme for carbon brokers and carbon credit bankers to make a buck imho.