Last year I inquired how one might take sea passage across the Atlantic. The realistic answer is that there are various ocean liners and transatlantic cruises traveling multiple times per year, none faster than 7 days and many taking closer to 10 or 14. Repositioning cruises, when the ship is being moved from one region to another and you just tag along, are infrequent but the cheapest option.

In February, I will unavoidably be in the United Kingdom. I am flying there, but have not purchased a flight back yet. I am thinking about taking a ship in repositioning from England to the US Eastern Seaboard as there is very conveniently one such ship leaving a couple days after my event is over. That would probably be Southampton to Miami (from there, I would take a train home) and would take 11 or so days, zero of which are at intermediary ports.

I was thinking about a ship over a plane because the last time I flew it was like my ears got blown out for an entire day afterward. I don't know what it is about my sinuses but they have never handled flying well. My height makes the experience particularly cramped and unpleasant and I have gotten sick on every plane I have taken in the last six months. It's the most miserable thing I do to myself on a regular basis.

Unfortunately, all the research I can find on passenger ship emissions seem to indicate that it is worse for the environment on a passenger-mile basis than flying that same route, at least as far as cruise ships are concerned (there is zero research on emissions from being a passenger on a container ship). As of 2006, the Queen Mary 2 ocean liner supposedly emits about 0.43kg CO2e per passenger-mile, compared to 0.257kg CO2e for a long-haul airplane. But emissions estimates vary so ridiculously widely that it is a little bit hard for me to take these figures seriously:

Emissions factors for individual journeys by cruise ships to or from New Zealand in 2007 ranged between 250 and 2200 g of CO2 per passenger-kilometre (g CO2 per p-km), with a weighted mean of 390 g CO2 per p-km.

That's literally an order of magnitude. I think there is some guessing going on here. To translate from p-km to p-mi, that's ~402–3545g CO2e/p-mile or a weighted mean of 628g CO2e/p-mi. I would speculate that a repositioning cruise (which spends no time in intermediary ports because it is specifically supposed to get somewhere efficiently) would be on the lower end of the spectrum. So, honestly, while worse than an airplane in terms of gaseous emissions, it's not... that much worse. From the way articles seem to talk about cruise emissions, I would have thought it would be at least an order of magnitude. (For reference, the difference between a train and a plane is about an order of magnitude.) Which I guess it can be based on the higher figures there, but I am pretty sure that that is derived from non-direct routings (port visits apparently contribute massively to emissions) or from luxury behaviors (i.e. having an enormous stateroom and other amenities that decrease space efficiency).

One may notice that those sources are almost 20 years old. There is more recent research on cruise ship emissions, but the non-academic stuff all seems to cite the Queen Mary 2 statistic (not sure where it even originated). I attempted to discern what cruise ship emissions looked like 20 years ago versus today, but was unable to find any specific information about passenger-mile emissions year-by-year. I don't have institutional access to most journals anymore, so feel free to share if you know anything.

There have definitely been new environmental regulations since 2007. In 2020, some new regulations limiting high-sulphur fuels went into effect. But sulfur dioxide is more of a health concern; it isn't a greenhouse gas. It's toxic to marine life too, and all other life, but wouldn't be considered in a passenger-mile CO2e emissions figure. Apparently the regulation has encouraged more ships to switch to less toxic liquefied natural gas (LNG) fuels rather than the literal bottom-of-the-barrel sludges they've traditionally burned (maybe a 20%-ish improvement at face value), but most ships have just installed scrubbers to continue using the same fuel and emit fewer horrible particulates. Apparently a switch to LNG, while favorable for human health, does not really reduce GHG emissions due to increased methane output.

Aakko-Saksa et al. 2023 seems to be the most comprehensive journal article I can access that covers current strategies in reducing greenhouse gas emissions for ship engines now and in the future. It mainly talks about fuels and technical stuff about engines I don't understand. This paper remarks that the switch to LNG could still be positive; it suggests a 30% reduction in GHG emissions compared to diesel fuel but a 6–23% reduction depending on how much "methane slip" happens; the IEA thinks it's 10% or less. That's still a meaningful reduction, though there is quite a lot of variance. The authors' takeaway is that there could be a significant benefit to switching to LNG and then blending that with greener fuels at increasing proportions over time.

There are many proposed ways to decarbonize the industry. It is not clear to me which of these have been adopted recently. The industry seems to have some interest in decarbonizing, or giving the appearance of having interest, as Norwegian claims to want to "reduce GHG intensity by 10% by 2026 and 25% by 2030, compared to a 2019 baseline with intensity measured on a per Capacity Day basis." And they are actually thinking about it: they have ESG staff and a 2022 ESG Report lays out a few relatively specific and achievable metrics (or so it seems to me, a layperson and a landlubber):

We were very excited to announce in early 2023 that two of our Norwegian Cruise Line newbuilds, expected to be delivered in 2027 and 2028, will be re-configured to accommodate the future use of green methanol. Green methanol is a fuel that we see as a promising future solution. Compared to conventional fuels, it can reduce carbon dioxide emissions by up to 95%, nitrogen oxide emissions by up to 80%, and all sulfur oxide and particulate matter emissions.

Environmental goals:

• Reduce GHG intensity by 10% by 2026 and by 25% by 2030, compared to 2019 baseline, and pursue net zero GHG emissions by 2050
• Decrease fleet-wide fuel consumption of boilers per day by 2% annually, compared to 2016
• 100% of fleet equipped with Waste Heat Recovery by 2027
• Increase the percentage of our fleet with shore power capabilities to 50% by 2024, 70% by 2025, and 100% by 2035
• Increase the percentage of treated wastewater compared to untreated sewage discharged by 2024, compared to 2019
• Reduce bunkering by 4%, as compared to 2019, by 2025
• Decrease the total volume of sludge offloaded fleetwide by 5%, compared to 2018, by 2023

Green methanol is apparently a real thing. The figures Norwegian uses are lifted directly from the Methanol Institute. It does seem like methanol production capacity is increasing and is on track to continue increasing, according to this source. If a cruise ship uses about 250 tons of fuel per hour (91250/year), and current green methanol production is just shy of 1 million tons/year, then if all of that production were directed toward cruise ships then it could fuel about... 11 ships. Hmm. If by 2027 production increases (generously) by an order of magnitude, that's still only about 100 ships, or less than 1/3 of the total fleet worldwide across all cruise firms. Better than nothing...?

Norwegian claims to be "on track" for all of these metrics, especially fuel consumption, but of course they will say that. I can't really figure out what their "-50%" and "-80%" figures mean and I suspect they mean nothing. Frankly most of these commitments are ridiculously insufficient, though it isn't reasonable to expect revolutionary changes to happen in just a couple years. I think we should take these commitments with a grain of salt, but it does make sense from an operational perspective why industry would be interested in improving their own efficiency, especially to avoid potentially crippling regulations from governments who they can clearly see are honing in on emissions.

From what I can tell, many or most of the emissions are not just from the transportation itself (i.e. the burning of fuel for the purpose of moving mass from point A to B) but rather from the hoteling aboard the ship (12x more emissions than land-based hoteling) and from other luxury-related activities. Intuitively, if we know how energy-efficient it is to transport goods by sea, this should come as no surprise: if we're comparing fuel costs, it simply takes less fuel to move objects by water than by air. This is why ferries have so few emissions per passenger-mile. Ship fuel is particularly nasty stuff as far as human health is concerned, but many resources appear to primarily emphasize the non-fuel waste produced by these enterprises. So the CO2e emissions of cruise ships would seem to originate not just from fuel but rather from the inefficiency of human habitation at sea. Norwegian is at least vaguely calculating multi-scope emissions (p. 15) with, for example, "purchased goods and services" apparently making up ~21% of total emissions in 2022 (fuel itself is about 55%, and "fuel and energy-related activities," whatever that means, being another 12%). I assume "capital goods" (18% of their emissions) are the emissions from the ships themselves, which is more of a decarbonization question for manufacturers.

There are also significant non-GHG environmental impacts due to operational procedures taken by cruise ships. Wikipedia has a whole page on the environmental effects of shipping (not just cruise ships, but they are included). Waste dumping, noise pollution, etc. Those externalities are different than the externalities produced by airplanes; same idea, but apples to oranges, so I don't know how to compare them.

Anyway, this is all to say: greenhouse gas emissions from cruise ships are pretty rough. Given the relative lack of information on repositioning cruises specifically, the age of much of the data, and newer emissions reductions which are maybe not yet reflected in the literature, I am going to speculate that such a trip has an approximately equal GHG impact as a long-haul flight, assuming a typical stateroom and a direct voyage. On average, it probably works out to somewhat more emissions, though I personally think ships have a clearer (easier/faster) path toward net-zero (ish) emissions than airplanes given the limitations of each mode.

I will go to sleep and decide tomorrow whether I will fly or sail home, but right now I am leaning toward the sea for this occasion. I am not sure about future voyages yet. We will see.