What is the lowest-impact regular¹ method to cross the Atlantic Ocean? Specifically, which method has the lowest carbon dioxide equivalent emission per passenger-kilometre? I'm aware there are other impacts to consider, but for this question I'm focussing on the carbon equivalent.

I thought a ship would be lower impact than a flight, but this appears to be false, at least when comparing to cruise ships. But what about freighters? They're slower, lack the luxuries of a cruise ship, and for freight there is little alternative. How does this affect the equation of the additional carbon footprint per passenger-kilometre?

See also: Impact of various travelling options for a more generic question on a similar topic.


¹By regular, I mean a way accessible for people without special skills or access to their own boats. Therefore, I'm not counting rowing, sailing or (until I can buy a ticket on one) solar boats.

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    I've heard from quite some people that start their transatlantic journey on sailing boats from Gran Canaria (usually going to the Caribbean), somewhat in the range of 100 - 200 people / year. One of them stated that this is easier than going by cargo vessels as those require insurance documents for taking passengers or workers. – Stockfisch Apr 29 '13 at 11:16
up vote 10 down vote accepted

If we diverge slightly from the request for CO_2 per passenger km, and look at energy use per passenger km, then David MacKay's book "Without Hot Air" has a rather good chart.

Pulling from that the methods by which one might plausibly cross the atlantic (figures are approximate, as I'm reading them off the vertical scale):

  • A Boeing 747: 52 kWh per 100 passenger km (42 if absolutely full)
  • An ocean liner: 120kWh per 100 passenger km (102 if full)
  • A private jet: 150kWh per 100 passenger km

It's a little unfair to compare a cruise ship to aeroplanes, because the energy used by the ship is not just for propulsion but also the passengers' hotel needs for a number of days - but the majority of it does go into moving the vessel.

So in pure energy terms, out of those options, one must choose the large airliner.

Where do freight ships fit into this? They're not shown on MacKay's chart, presumably because they're not normally a means of transporting passengers. However, one of the reasons that passenger ships burn a lot of fuel is that they go fast - because people want to get places quickly. By contrast, sea freight tends to be carried at something much closer to the most economical speed (and getting ever closer to this, as rising fuel costs prompt shipping lines to introduce "slow steaming"). The very fact that most freight is carried by sea rather than by air suggests that it must be more energy efficient.

In fact MacKay does cover freight transport with another chart, this time in terms of energy use per tonne-km of freight. From this chart,

  • Sea: 0.02-0.15 kWh/tonne-km
  • Air: 1.65 kWh/tonne-km

Note that this book is some years old, and that the figures for sea freight do not allow for slow steaming; if produced today, it would look even more advantageous for the ships. It's reasonable to conclude, then, that taking passage on board a freight ship across the atlantic requires at least an order of magnitude less energy than flying - so long as you're not in a hurry!

If thinking about CO_2 rather than energy use, then air travel is further disadvantaged. Jet fuel is a lighter oil than heavy fuel oil, and so probably produces a little less CO_2 per joule of energy provided, but what it does produce is delivered straight into the upper atmosphere, as noted in comments to another answer that link to this paper.

Note also that there are many other aspects that are relevant to just the pollution from these modes of travel, let alone the broader sustainability, such as NO_x, SO_2 and particulate emissions, which vary between transport types.

I don't have the expertise to quantify those effects, but in energy terms the methods of crossing the atlantic from the least energy required to the most are,

  1. Freight vessel
  2. Fully laden airliner
  3. Passenger ship
  4. Private jet
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    I like that you take a different view on this, but I also think that energy efficiency can be rather misleading (or at least intransparent) when it comes to sustainability and/or emissions. The energy used by freighter ships usually comes from low-quality bunkerfuel, which is the worst kind of fuel if you look at CO2, sulfur and microparticle emissions. – THelper Jan 16 '14 at 8:47
  • @THelper that's very true, and the same is true of most (not all) passenger liners (although sulphur is improving with regulation). I'll edit to make it clearer that this answer only considers energy, and that there are other relevant factors. – Flyto Jan 16 '14 at 9:14

Let's assume that the use of fuel is the key factor to contribute to global warming and identify four ways that an individual can choose to cross the Atlantic ocean:

Large passenger ships, small passenger ships (sailing boats), large cargo vessels and airplane.

Fuel use per transport mode

Here are the km a given transport mode delivers per one litre of fuel:

¹ I have no idea if you can comfortably cross the Atlantic in a 10 m sailing boat and neither how many passengers (including crew) can be fitted. Just a guess (please correct me here): 5 people.

Fuel used and greenhouse gas emission

  • Big shipping vessels (cruise ships, cargo vessels) use bunker fuel or heavy fuel oil. CO2 emission = 3.17² kg / kg fuel.
  • Small ships (fishing boats, sailing boats) use marine diesel (also "red diesel" / "gas oil") which is very similar to road diesel. CO2 emission = 3.19² kg / kg fuel.
  • Airplanes use kerosene. CO2 emission = 3.15² kg / kg fuel.

There are very small methane emissions in the order of less than 0.1% for all modes of transport which are ignored for now.

² Source of CO2 and CH4 emissions: GaBi life cycle assessment software database.

CO2 emissions per kg of fuel and mode of transport

This leads us to a CO2 emission per one kg of fuel per km and passenger

  • Average airplane: 0.15³
  • Cruise ship: 0.54
  • Sailing boat (if engine is used): 0.25
  • Cargo vessel: 0.001 - loading each 40 ft container with 56 people, giving them half a square meter of space (seriously?)

³ This number does not take into account the impact of CO2 emitted at high altitude (as @EnergyNumbers pointed out in the comments).

To conclude

The results depend mainly on your assumptions but to keep it simple; if the motivation for the crossing is to transport cargo I believe it is the way with the least CO2 emissions because "they will go anyway". In case you manage to not use any fuel on a sailing boat this will be the way of choice. Any sailing boat with the given specifications will definitely not use as much fuel as stated because the sail will be used most of the time. This mode of transport will then rank second, before flying and going by cruise ship.

Consider the numbers in here as very rough approximation as so many factors are neglected or just one sample per transport mode is taken into account.

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    There is a multiplier that gets applied to aircraft emissions: do your calculations include that? – EnergyNumbers May 19 '13 at 10:18
  • The CO2 emissions are sheer numbers per one kg of fuel combusted. What is this multiplier about and to which emission is it assigned? – Stockfisch May 19 '13 at 12:26
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    It's a factor to account for the peculiar nature of aviation emissions: water vapour, NOx and CO2 at altitude. See for example Transport impacts on atmosphere and climate: Aviation, Lee et al 2010. There is a range of numbers used for the multiplier in the literature: 2-5, IIRC. This might be worth a question in it's own right – EnergyNumbers May 19 '13 at 12:44
  • No, this is not taken into account and at the moment I'm not willing to dive into that :) .. there are so many more factors to be considered for a thorough answer and I clearly lack the knowledge (and the time) to assess most of those. I hope my answer will still be perceived as an attempt to answer the question. – Stockfisch May 19 '13 at 13:54

It's hard to assume how much ecological footprint does one extra passenger add to a freighter. I'd guess it's near zero, since it's a cargo ship which needs some basic facilities for the crew anyway, so one person doesn't mean a difference. The twelve passengers limit for cruisers without a doctor means freighter travel isn't likely to be mass used, but for an individual travel it seems more sustainable than anything else (whether transport of anything on such distances is sustainable is another question).

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    I would also note that some places which offer freighter travel also offer specific sailing crossings between the Caribbean and Gibraltar, so I wouldn't rule sailing ships out! – Chris Travers Apr 30 '13 at 5:27

Unfortunately--and I realize this is not very helpful--the best choice in terms of carbon footprint is almost certainly not to go. It's a good point that freighters are likely to be much more efficient per passenger than cruise ships, but when comparing them to airplanes, you have to keep in mind what's being brought along to accommodate the passenger, not just what's being expended to move the passenger personally.

On airplanes, although shared facilities (the body of the airplane itself, the aisle, the rest rooms, the flight attendants and their seats, the galley, etc.) are substantial, each additional passenger's main impact is the passenger and the seat that passenger sits in. On an oceangoing vessel, you're transporting not just the passenger, but also the room the passenger stays in and all its furnishings. To help put this in perspective, imagine you were to order an entire room, including walls, bed, chairs, etc., from Europe and have it delivered to an American port (or vice-versa).

It could be argued that the room would be transported whether or not the passenger was there, but if there were no passengers, in theory the room could be converted into additional cargo space, or never built in the first place. The reason it's there and being transported around is because there are passengers interested in traveling in it.

Unfortunately, I have no idea what the impact of transporting a room across the ocean on a freighter is. Perhaps it's not as enormous as I imagine. However, if it's anywhere in the ballpark of the impact of a transatlantic flight, it's literally tons of CO2 equivalent and therefore many times, for instance, the entire annual carbon footprint of a person living in one of the poorer countries of the world.

There's a huge carbon footprint just in growing the food you'll eat on the journey.

Fun fact: An airliner gets roughly 30 mpg/passenger -- the same as a compact car. The compact car only beats the airliner if you make the trip with passengers.

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    Do you have any references for the 30 mpg/passenger? – THelper May 17 '13 at 5:31
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    I calculated it once on my own and came up with 30. This Wikipedia page: en.wikipedia.org/wiki/Fuel_economy_in_aircraft says that a modern airliner gets 75 miles/gallon/passenger, which is a lot better than the number I came up with. – Edward Falk May 17 '13 at 6:23
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    Is eating food on a ship worse than eating food off a ship? – gerrit May 18 '13 at 4:39
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    @gerrit Probably slightly as is uses fuel to take it along and refrigeration is likely to be not so environmentally friendly compared to land. I believe the impact is very negligible though. – Stockfisch May 18 '13 at 22:28
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    As far as the food situation goes, it depends on the purpose of the trip. If you're on vacation, the purpose is to kill time and enjoy yourself. You would have spent the same time and eaten the same food either way, so you wouldn't count the food against the carbon budget unless there was some reason why food onboard a ship had a higher footprint than food at the vacation spot. However, if the purpose of the trip is to get from point A to point B, then the extra time it takes to go by ship does count against the carbon footprint. – Edward Falk Jul 4 '14 at 1:41

From the North East USA, get to St John's Newfoundland. Fly 4 hours on Westjet to Dublin Ireland (shortest transatlantic flight). Minimize flying time, taking train from NYC or Boston to Montreal, train to Halifax, bus and passenger ferry to Newfoundland.

Trains are considered the most energy-efficient means of transport and are way more energy-efficient than airplanes. That is why people make the latter the main means of long-distance travel. Moreover, the Transatlantic Train (see Wikipedia or YouTube) is going to travel in a vacuum tunnel to reduce the friction/leakage to practically zero. The only energy waste would be to heat up the pot (to accelerate your vehicle). But to speed up, you can use the energy of de-accelerating trains at arrival, so you can transport any mass virtually for free. Once the Transatlantic tunnel is built, I'd advice to use that.

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    Thanks, but I was looking for solutions that exist now, not proposals that are unlikely to emerge in my lifetime. – gerrit May 6 '16 at 12:38
  • They say that the tunnel is pretty practical and can be built right now. Moreover, you are likely to experience the peak oil in your lifetime and the tunnel will remain the only practical means of massive trans-atlantic travel. So, you better consider this opportunity if you are serious about it. – Little Alien May 6 '16 at 12:47
  • Maybe it can be, or maybe not, but it isn't there right now and nobody is currently building or concretely planning to build it either. Therefore, it's not the lowest impact transatlantic crossing right now. If I'm proven wrong and it's getting built in the next 50 years, and if it is by then the lowest-impact transatlantic crossing (who knows how the alternatives develop), I will accept your answer ;-) – gerrit May 6 '16 at 12:51
  • Which technologies? Teleporation or traveling with 0 friction in the air? – Little Alien May 6 '16 at 13:37

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